Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs
This part is a continuation of the Part I where we built resolutions of identity for certain non-Hermitian Hamiltonians constructed of biorthogonal sets of their eigen- and associated functions for the spectral problem defined on entire axis. Non-Hermitian Hamiltonians under consideration are taken...
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irk-123456789-1480892019-02-17T01:23:10Z Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs Sokolov, A.V. This part is a continuation of the Part I where we built resolutions of identity for certain non-Hermitian Hamiltonians constructed of biorthogonal sets of their eigen- and associated functions for the spectral problem defined on entire axis. Non-Hermitian Hamiltonians under consideration are taken with continuous spectrum and the following cases are examined: an exceptional point of arbitrary multiplicity situated on a boundary of continuous spectrum and an exceptional point situated inside of continuous spectrum. In the present work the rigorous proofs are given for the resolutions of identity in both cases. 2011 Article Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs / A.V. Sokolov // Symmetry, Integrability and Geometry: Methods and Applications. — 2011. — Т. 7. — Бібліогр.: 4 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 81Q60; 81R15; 47B15 DOI: http://dx.doi.org/10.3842/SIGMA.2011.112 http://dspace.nbuv.gov.ua/handle/123456789/148089 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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This part is a continuation of the Part I where we built resolutions of identity for certain non-Hermitian Hamiltonians constructed of biorthogonal sets of their eigen- and associated functions for the spectral problem defined on entire axis. Non-Hermitian Hamiltonians under consideration are taken with continuous spectrum and the following cases are examined: an exceptional point of arbitrary multiplicity situated on a boundary of continuous spectrum and an exceptional point situated inside of continuous spectrum. In the present work the rigorous proofs are given for the resolutions of identity in both cases. |
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Sokolov, A.V. Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs Symmetry, Integrability and Geometry: Methods and Applications |
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Sokolov, A.V. |
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Sokolov, A.V. |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs |
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resolutions of identity for some non-hermitian hamiltonians. ii. proofs |
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Resolutions of Identity for Some Non-Hermitian Hamiltonians. II. Proofs / A.V. Sokolov // Symmetry, Integrability and Geometry: Methods and Applications. — 2011. — Т. 7. — Бібліогр.: 4 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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AT sokolovav resolutionsofidentityforsomenonhermitianhamiltoniansiiproofs |
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2025-07-12T18:12:36Z |
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1837465821960470528 |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 7 (2011), 112, 16 pages
Resolutions of Identity for Some Non-Hermitian
Hamiltonians. II. Proofs?
Andrey V. SOKOLOV
V.A. Fock Department of Theoretical Physics, Saint-Petersburg State University,
198504 Saint-Petersburg, Russia
E-mail: avs avs@rambler.ru
Received August 06, 2011, in final form November 25, 2011; Published online December 05, 2011
http://dx.doi.org/10.3842/SIGMA.2011.112
Abstract. This part is a continuation of the Part I where we built resolutions of iden-
tity for certain non-Hermitian Hamiltonians constructed of biorthogonal sets of their eigen-
and associated functions for the spectral problem defined on entire axis. Non-Hermitian
Hamiltonians under consideration are taken with continuous spectrum and the following
cases are examined: an exceptional point of arbitrary multiplicity situated on a boundary
of continuous spectrum and an exceptional point situated inside of continuous spectrum. In
the present work the rigorous proofs are given for the resolutions of identity in both cases.
Key words: non-Hermitian quantum mechanics; supersymmetry; exceptional points; resolu-
tion of identity
2010 Mathematics Subject Classification: 81Q60; 81R15; 47B15
1 Introduction
This part is a continuation of the Part I [1] where resolutions of identity for certain non-Hermitian
Hamiltonians were constructed of biorthogonal sets of their eigen- and associated functions. The
spectral problem was defined on entire axis. Non-Hermitian Hamiltonians were taken with con-
tinuous spectrum and they were endowed with an exceptional point of arbitrary multiplicity
situated on a boundary of continuous spectrum or an exceptional point situated inside of con-
tinuous spectrum. In the present work (Part II) the detailed rigorous proofs are given for the
resolutions of identity in both cases. Moreover the reductions of the derived resolutions of iden-
tity under narrowing of the classes of employed test functions in the Gel’fand triple [2] are built.
In Section 2 the definitions of the employed spaces of test functions and distributions are given.
In Section 3 the proofs of the initial resolution of identity and of its reduced forms for restricted
spaces of test functions are elaborated for an exceptional point of arbitrary multiplicity situated
on a boundary of continuous spectrum. In Section 4 the analogous proofs of resolutions of
identity are presented for an exceptional point situated inside of continuous spectrum.
2 Definition of spaces of test functions and distributions
In this paper we shall use the following spaces of test functions and distributions.
Let CLγ = C∞R ∩ L2(R; (1 + |x|)γ), γ ∈ R, be the space of test functions. The sequence
ϕn(x) ∈ CLγ , n = 1, 2, 3, . . . is called convergent in CLγ to ϕ(x) ∈ CLγ ,
limγ
n→+∞
ϕn(x) = ϕ(x)
?This paper is a contribution to the Proceedings of the Workshop “Supersymmetric Quantum Me-
chanics and Spectral Design” (July 18–30, 2010, Benasque, Spain). The full collection is available at
http://www.emis.de/journals/SIGMA/SUSYQM2010.html
mailto:avs_avs@rambler.ru
http://dx.doi.org/10.3842/SIGMA.2011.112
http://www.emis.de/journals/SIGMA/SUSYQM2010.html
2 A.V. Sokolov
if
lim
n→+∞
∫ +∞
−∞
|ϕn(x)− ϕ(x)|2(1 + |x|)γdx = 0,
and for any x1, x2 ∈ R, x1 < x2 and any l = 0, 1, 2, . . . ,
lim
n→+∞
max
[x1,x2]
∣∣ϕ(l)
n (x)− ϕ(l)(x)
∣∣ = 0.
We shall denote the value of a functional f on ϕ ∈ CLγ conventionally as (f, ϕ). A linear
functional f is called continuous if for any sequence ϕn ∈ CLγ , n = 1, 2, 3, . . . convergent in
CLγ to zero the equality
lim
n→+∞
(f, ϕn) = 0
is valid. The space of distributions over CLγ , i.e. of linear continuous functionals over CLγ , is
denoted CL′γ . The sequence fn ∈ CL′γ , n = 1, 2, 3, . . . is called convergent in CL′γ to f ∈ CL′γ ,
lim′γ
n→+∞
fn = f,
if for any ϕ ∈ CLγ the relation takes place,
lim
n→+∞
(fn, ϕ) = (f, ϕ).
A functional f ∈ CL′γ is called regular if there is f(x) ∈ L2(R; (1 + |x|)−γ) such that for any
ϕ ∈ CLγ the equality
(f, ϕ) =
∫ +∞
−∞
f(x)ϕ(x) dx
holds. In this case we shall identify the distribution f ∈ CL′γ with the function f(x) ∈ L2(R; (1+
|x|)−γ). In virtue of the Bunyakovskii inequality,∣∣∣∣∫ +∞
−∞
f(x)ϕ(x) dx
∣∣∣∣2 6 ∫ +∞
−∞
|f2(x)| dx
(1 + |x|)γ
∫ +∞
−∞
∣∣ϕ2(x)
∣∣(1 + |x|)γ dx,
it is evident that L2(R; (1 + |x|)−γ) ⊂ CL′γ and this inclusion is continuous.
For any γ1 < γ2 there is a continuous inclusion CLγ2 ⊂ CLγ1 . Let us also notice that the
Dirac delta function δ(x− x′) belongs to CL′γ for any γ ∈ R.
3 Proofs of resolutions of identity for the model Hamiltonians
with exceptional point of arbitrary multiplicity
at the bottom of continuous spectrum
3.1 Proof of the biorthogonality relations between eigenfunctions
for continuous spectrum
Let us start proofs by proving of the biorthogonality relation∫ +∞
−∞
[
knψn(x; k)
][
(k′)nψn(x;−k′)
]
dx = (k′)2nδ(k − k′) (3.1)
between eigenfunctions ψn(x; k) for the continuous spectrum of the Hamiltonian hn, n = 0, 1,
2, . . . (see (2.17) of Part I). Proof of this biorthogonality relation (3.1) is based on the following
Lemmas 3.1–3.3.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 3
Lemma 3.1. Suppose that the functions ψn(x; k), n = 0, 1, 2, . . . are defined by the formula (2.6)
of Part I for any x ∈ R, k ∈ C, k 6= 0 and fixed z ∈ C, Im z 6= 0. Then for any n = 1, 2, 3, . . . ,
R > 0, k ∈ C and k′ ∈ C the following relation holds,∫ R
−R
[
knψn(x; k)
][
(k′)nψn(x;−k′)
]
dx = (k′)2n
sinR(k − k′)
π(k − k′)
− i
n−1∑
l=0
(k′)2l
[
kn−1−lψn−1−l(x; k)
][
(k′)n−lψn−l(x;−k′)
]∣∣∣R
−R
. (3.2)
Proof. Let us check first that∫ R
−R
[
knψn(x; k)][(k
′)nψn(x;−k′)
]
dx = −i
[
kn−1ψn−1(x; k)
][
(k′)nψn(x;−k′)
]∣∣∣R
−R
+ (k′)2
∫ R
−R
[
kn−1ψn−1(x; k)
][
(k′)n−1ψn−1(x;−k′)
]
dx. (3.3)
This equality can be derived with the help of (2.3), (2.6) and (2.9) of Part I and integration by
parts,∫ R
−R
[
knψn(x; k)
][
(k′)nψn(x;−k′)
]
dx
= i
∫ R
−R
{(
−∂ +
n
x− z
)
[kn−1ψn−1(x; k)]
}[
(k′)nψn(x;−k′)
]
dx
= −i
[
kn−1ψn−1(x; k)
][
(k′)nψn(x;−k′)
]∣∣∣R
−R
+ i
∫ R
−R
[
kn−1ψn−1(x; k)
]{(
∂ +
n
x− z
)[
(k′)nψn(x;−k′)
]}
dx
= −i
[
kn−1ψn−1(x; k)
][
(k′)nψn(x;−k′)
]∣∣∣R
−R
+
∫ R
−R
[
kn−1ψn−1(x; k)
]{
q−n q
+
n
[
(k′)n−1ψn−1(x;−k′)
]}
dx
= −i
[
kn−1ψn−1(x; k)
][
(k′)nψn(x;−k′)
]∣∣∣R
−R
+
∫ R
−R
[
kn−1ψn−1(x; k)
]{
hn−1
[
(k′)n−1ψn−1(x;−k′)
]}
dx
= −i
[
kn−1ψn−1(x; k)
][
(k′)nψn(x;−k′)
]∣∣∣R
−R
+ (k′)2
∫ R
−R
[
kn−1ψn−1(x; k)
][
(k′)n−1ψn−1(x;−k′)
]
dx.
The equality (3.2) follows from (3.3) by induction, in view of the relation∫ R
−R
ψ0(x; k)ψ0(x;−k′) dx =
1
2π
∫ R
−R
eix(k−k
′) dx =
sinR(k − k′)
π(k − k′)
.
Lemma 3.1 is proved. �
Lemma 3.2. For any k′ ∈ R, z ∈ C, Im z 6= 0, j = 0, 1, 2, . . . , l = 0, 1, 2, . . . , m = 0, 1, 2, . . .
and γ > 1 + 2l − 2m the following relation holds,
lim′γ
x→±∞
kleix(k−k
′)
(1 + k2)m/2(x− z)j
= 0.
4 A.V. Sokolov
Proof. It is true that
kleix(k−k
′)
(1 + k2)m/2(x− z)j
∈ L2(R; (1 + |k|)−γ) ⊂ CL′γ , γ > 1 + 2l − 2m.
Thus, to prove Lemma 3.2, it is sufficient to prove that for any ϕ(k) ∈ CLγ the function
klϕ(k)/(1+ k2)m/2 belongs to L1
R in view of the Riemann theorem. The latter is valid by virtue
of the Bunyakovskii inequality:(∫ +∞
−∞
|klϕ(k)|
(1 + k2)m/2
dk
)2
6
∫ +∞
−∞
|ϕ2(k)|(1 + |k|)γ dk
∫ +∞
−∞
k2l dk
(1 + k2)m(1 + |k|)γ
< +∞,
where the condition γ > 1 + 2l − 2m is taken into account. Lemma 3.2 is proved. �
Corollary 3.1. In the conditions of Lemma 3.1, in view of (2.6) of Part I by virtue of Lemma 3.2
for any m = 0, 1, 2, . . . , n = 1, 2, 3, . . . , l = 0, . . . , n−1 and γ > −2l−2m+2n−1, the following
relation holds,
lim′γ
x→±∞
(k′)2l
[
kn−1−lψn−1−l(x; k)
(1 + k2)m/2
] [
(k′)n−lψn−l(x;−k′)
(1 + (k′)2)m/2
]
= 0.
Lemma 3.3. For any k′ ∈ R, m = 0, 1, 2, . . . , n = 0, 1, 2, . . . and γ > −2m − 1 the following
relation is valid,
lim′γ
R→+∞
(k′)2n
(1 + k2)m/2(1 + (k′)2)m/2
sinR(k − k′)
π(k − k′)
=
(k′)2n
(1 + (k′)2)m
δ(k − k′).
The proof of Lemma 3.3 is quite analogous to the one for Lemma 3.6 from Section 3.2.
Validity of the biorthogonality relation (3.1) is a corollary of the following theorem.
Theorem 3.1. Suppose that the functions ψn(x; k), n = 0, 1, 2, . . . are defined by the for-
mula (2.6) of Part I for any x ∈ R, k ∈ C, k 6= 0 and fixed z ∈ C, Im z 6= 0. Then for any
k′ ∈ R, m = 0, 1, 2, . . . , n = 0, 1, 2, . . . and γ > −2m+2n− 1 the following relation takes place,
lim′γ
R→+∞
∫ R
−R
[
knψn(x; k)
(1 + k2)m/2
] [
(k′)nψn(x;−k′)
(1 + (k′)2)m/2
]
dx =
(k′)2n
(1 + (k′)2)m
δ(k − k′). (3.4)
The statement of Theorem 3.1 follows from Lemmas 3.1 and 3.3 and from Corollary 3.1.
Remark 3.1. The parameterm in Theorem 3.1 regulates the class of test functions for which the
biorthogonality relation (3.4) takes place. One can prove as well this relation for any fixed m
for test functions from a wider class than in Theorem 3.1 with the help of the technique of
Theorem 3.3 and Remark 3.2 from Section 3.2.
3.2 Proofs of the resolutions of identity
Proof of the initial resolution of identity (2.18) of Part I is based on the following Lemmas 3.4–3.6.
Lemma 3.4. Suppose that
(1) the functions ψn(x; k), n = 0, 1, 2, . . . are defined by the formula (2.6) of Part I for any
x ∈ R, k ∈ C, k 6= 0 and fixed z ∈ C, Im z 6= 0;
(2) L(A) is a path in complex k plane, made of the segment [−A,A] by its deformation near
the point k = 0 upwards or downwards and the direction of L(A) is specified from −A
to A.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 5
Then for any n = 1, 2, 3, . . . , x ∈ R and x′ ∈ R the following relation holds,∫
L(A)
ψn(x; k)ψn(x
′;−k) dk
=
n−1∑
l=0
(
x′ − z
x− z
)l ψn−1−l(x; k)ψn−l(x′;−k)
i(x− z)
∣∣∣A
−A
+
(
x′ − z
x− z
)n sinA(x− x′)
π(x− x′)
. (3.5)
Proof. Let us check first that∫
L(A)
ψn(x; k)ψn(x
′;−k) dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+
x′ − z
x− z
∫
L(A)
ψn−1(x; k)ψn−1(x
′;−k) dk. (3.6)
This equality can be derived with the help of (2.3), (2.6) and (2.9) of Part I and of integration
by parts:∫
L(A)
ψn(x; k)ψn(x
′;−k) dk =
∫
L(A)
eikz
i(x− z)
[(
∂
∂k
− n
k
)(
e−ikzψn−1(x; k)
)]
ψn(x
′;−k) dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
−
∫
L(A)
e−ikzψn−1(x; k)
i(x− z)
[(
∂
∂k
+
n
k
)(
eikzψn(x
′;−k)
)]
dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+ i
x′ − z
x− z
∫
L(A)
e−ikzψn−1(x; k)
x′ − z
[(
∂
∂k
+
n
k
)(
eikzψn(x
′;−k)
)]
dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+ i
x′ − z
x− z
∫
L(A)
e−ikzψn−1(x; k)
[(
1
k
∂
∂x′
+
n
k(x′ − z)
)(
eikzψn(x
′;−k)
)]
dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+
x′ − z
x− z
∫
L(A)
1
k2
ψn−1(x; k)
[(
∂
∂x′
+
n
x′ − z
)(
− ∂
∂x′
+
n
x′ − z
)
ψn−1(x
′;−k)
]
dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+
x′ − z
x− z
∫
L(A)
1
k2
ψn−1(x; k)
[
hn−1ψn−1(x
′;−k)
]
dk
=
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
+
x′ − z
x− z
∫
L(A)
ψn−1(x; k)ψn−1(x
′;−k) dk.
The equality (3.5) follows from (3.6) by induction in view of the relation∫
L(A)
ψ0(x; k)ψ0(x
′;−k) dk =
1
2π
∫ A
−A
eik(x−x
′) dk =
sinA(x− x′)
π(x− x′)
. (3.7)
Lemma 3.4 is proved. �
Lemma 3.5. For any x′ ∈ R, z ∈ C, Im z 6= 0, l = 0, 1, 2, . . . , m = 1, 2, 3, . . . and γ > 1− 2m
the following relation takes place,
lim′γ
k→±∞
eik(x−x
′)
kl(x− z)m
= 0.
6 A.V. Sokolov
Proof. It is true that
eik(x−x
′)
kl(x− z)m
∈ L2(R; (1 + |x|)−γ) ⊂ CL′γ , γ > 1− 2m.
Thus, in view of the Riemann theorem, in order to prove the lemma, it is sufficient to prove
that for any ϕ(x) ∈ CLγ the fraction ϕ(x)/(x− z)m belongs to L1
R. The latter is valid by virtue
of the Bunyakovskii inequality:(∫ +∞
−∞
|ϕ(x)|
|x− z|m
dx
)2
6
∫ +∞
−∞
|ϕ2(x)|(1 + |x|)γ dx
∫ +∞
−∞
dx
|x− z|2m(1 + |x|)γ
< +∞,
where the condition γ > 1− 2m is taken into account. Lemma 3.5 is proved. �
Corollary 3.2. In the conditions of Lemma 3.4, in view of (2.6) from Part I by virtue of
Lemma 3.5 for any n = 1, 2, 3, . . . , l = 0, . . . , n−1 and γ > −2l−1, the following relation holds,
lim′γ
k→±∞
(
x′ − z
x− z
)l ψn−1−l(x; k)ψn−l(x′;−k)
i(x− z)
= 0.
Lemma 3.6. For any x′ ∈ R, z ∈ C, Im z 6= 0, n = 0, 1, 2, . . . and γ > −2n − 1 the following
relation is valid,
lim′γ
A→+∞
(
x′ − z
x− z
)n sinA(x− x′)
π(x− x′)
= δ(x− x′).
Proof. It is true that(
x′ − z
x− z
)n sinA(x− x′)
π(x− x′)
∈ L2(R; (1 + |x|)−γ) ⊂ CL′γ , γ > −2n− 1.
Thus, to prove the lemma, it is sufficient to prove that for any ϕ(x) ∈ CLγ , γ > −2n − 1 the
equality
lim
A→+∞
∫ +∞
−∞
sinA(x− x′)
π(x− x′)
(
x′ − z
x− z
)n
ϕ(x) dx = ϕ(x′)
takes place. For this purpose let us consider the function
ψ(x) =
(
x′ − z
x− z
)n
ϕ(x).
By virtue of the Bunyakovskii inequality for arbitrary δ > 0,[(∫ x′−δ
−∞
+
∫ +∞
x′+δ
)
|ψ(x)|
|x− x′|
dx
]2
6
(∫ x′−δ
−∞
+
∫ +∞
x′+δ
)
|ϕ2(x)|(1 + |x|)γ dx
×
(∫ x′−δ
−∞
+
∫ +∞
x′+δ
)
|x′ − z|2n dx
|x− z|2n|x− x′|2(1 + |x|)γ
< +∞
the following inclusion is valid,
ψ(x)
x− x′
∈ L1(R \ (x′ − δ, x′ + δ)), δ > 0,
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 7
and, moreover, it is evident that
ψ(x)− ψ(x′)
x− x′
∈ L1([x′ − δ, x′ + δ]), δ > 0.
Hence, by virtue of the Riemann theorem,
lim
A→+∞
∫ +∞
−∞
sinA(x− x′)
π(x− x′)
(
x′ − z
x− z
)n
ϕ(x) dx
= lim
A→+∞
[
ψ(x′)
∫ x′+δ
x′−δ
sinA(x− x′)
π(x− x′)
dx
]
=
2
π
ϕ(x′)
∫ +∞
0
sin t
t
dt = ϕ(x′).
Thus, Lemma 3.6 is proved. �
Validity of the resolution of identity (2.18) of Part I in CL′γ for any γ > −1 is a corollary of
the following theorem.
Theorem 3.2. Suppose that
(1) the functions ψn(x; k), n = 0, 1, 2, . . . are defined by the formula (2.6) of Part I for any
x ∈ R, k ∈ C, k 6= 0 and fixed z ∈ C, Im z 6= 0;
(2) L(A) is a path in complex k plane, made of the segment [−A,A], A > 0 by its deformation
near the point k = 0 upwards or downwards and the direction of L(A) is specified from −A
to A.
Then for any γ > −1, x′ ∈ R and n = 0, 1, 2, . . . the following relation holds,
lim′γ
A→+∞
∫
L(A)
ψn(x; k)ψn(x
′;−k) dk = δ(x− x′).
The statement of Theorem 3.2 follows from Lemmas 3.4 and 3.6 and from Corollary 3.2.
The applicability of the resolution of identity (2.18) of Part I for some bounded and slowly
increasing test functions is based on the next theorem.
Theorem 3.3. Suppose that
(1) the functions ψn(x; k), n = 0, 1, 2, . . . are defined by the formula (2.6) of Part I for any
x ∈ R, k ∈ C, k 6= 0 and fixed z ∈ C, Im z 6= 0;
(2) L(A) is a path in complex k plane, made of the segment [−A,A], A > 0 by its deformation
near the point k = 0 upwards or downwards and the direction of L(A) is specified from −A
to A;
(3) the function η(x) ∈ C∞R , η(x) ≡ 0 for any x 6 1, η(x) ∈ [0, 1] for any x ∈ [1, 2] and
η(x) ≡ 1 for any x > 2.
Then for any κ ∈ [0, 1), k0 ∈ R, x′ ∈ R and n = 0, 1, 2, . . . the following relation is valid,
lim
A→+∞
∫ +∞
−∞
[∫
L(A)
ψn(x; k)ψn(x
′;−k) dk
] [
η(±x)eik0x|x|κ
]
dx = η(±x′)eik0x′ |x′|κ. (3.8)
Proof. In the case n = 0 in view of (3.7) the proof can be easily realized in the same way as
for Theorem 2 from Appendix B of [4]. Thus, we present the proof for the case n = 1, 2, 3, . . .
with upper signs in (3.8) only, taking into account that the proof for the case with lower signs
8 A.V. Sokolov
is quite similar. In order to prove Theorem 3.3 in this case we employ Lemmas 3.4 and 3.6,
Corollary 3.2 and the fact that
η(x)eik0x|x|κ ∈ CLγ , −3 < γ < −1− 2κ. (3.9)
Then it is sufficient to prove that
lim
A→+∞
∫ +∞
−∞
[
ψn−1(x; k)ψn(x
′;−k)
i(x− z)
∣∣∣A
−A
] [
η(x)eik0x|x|κ
]
dx = 0.
In turn, to prove the latter, in view of (2.6) from Part I, (3.9) and Lemma 3.5, it is sufficient to
prove that
lim
A→+∞
∫ +∞
−∞
[
e±iA(x−x
′)
x− z
] [
η(x)eik0x|x|κ
]
dx = 0. (3.10)
The equality (3.10) follows from the Riemann theorem and the chain of transformations,∫ +∞
−∞
[
e±iA(x−x
′)
x− z
] [
η(x)eik0x|x|κ
]
dx = eik0x
′
∫ +∞
−∞
|t+ x′|κη(t+ x′)
t+ x′ − z
ei(k0±A)t dt
= eik0x
′
∫ +∞
−∞
|t+ x′|κη(t+ x′)
t+ x′ − z
d
ei(k0±A)t
i(k0 ±A)
= − eik0x
′
i(k0 ±A)
∫ +∞
−∞
ei(k0±A)td
|t+ x′|κη(t+ x′)
t+ x′ − z
, A > |k0|,
derived with help of integration by parts. Thus, Theorem 3.3 is proved. �
Remark 3.2. Theorems 3.2 and 3.3 provide the validity of the resolution of identity (2.18)
from Part I for test functions which are linear combinations of functions η(±x)eik0x|x|κ, in
general, with different κ ∈ [0, 1) and k0 ∈ R and functions from CLγ , in general, with different
γ > −1. In particular, these theorems guarantee applicability of (2.18) from Part I for the
eigenfunctions (2.6) from Part I and for the associated function (2.2) from Part I (in the case of
even n) of the Hamiltonian hn, n = 0, 1, 2, . . . .
Remark 3.3. The first of resolutions of identity (2.19) of Part I follows from (2.18) of Part I
and Lemma 3.4. The second of resolutions of identity (2.19) of Part I and (2.35) of Part I can
be derived from the first of resolutions of identity (2.19) of Part I with the help of calculation of
the substitution |ε−ε and of identical transformations. The resolution of identity (2.20) of Part I
follows trivially from (2.19) of Part I.
The resolutions of identity (2.21) and (2.36) of Part I are corollaries of the resolutions of
identity (2.19) and (2.35) of Part I respectively and of the following Lemma 3.7.
Lemma 3.7. For any γ > −1 and x′ ∈ R the relation holds,
lim′γ
ε↓0
sin ε(x− x′)
x− x′
= 0.
Proof of Lemma 3.7 is analogous to the proof of Lemma 2 from Appendix B of [4].
The resolution of identity (2.37) of Part I is a corollary of the resolution of identity (2.36) of
Part I and of the following Lemma 3.8.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 9
Lemma 3.8. For any γ > 1, x′ ∈ R and z ∈ C, Im z 6= 0 the relation takes place,
lim′γ
ε↓0
sin2 ε2(x− x
′)
ε(x− z)(x′ − z)
= 0.
Proof of Lemma 3.8 is analogous to the proof of Lemma 3 from Appendix of [3].
The resolution of identity (2.38) of Part I is a corollary of the resolution of identity (2.37) of
Part I and of the following Lemmas 3.9 and 3.10.
Lemma 3.9. For any γ > 3, x′ ∈ R and z ∈ C, Im z 6= 0 the relation is valid,
lim′γ
ε↓0
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
= 0.
Proof. It is true that
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
∈ L2(R; (1 + |x|)−γ) ⊂ CLγ , γ > 3.
Thus, to prove the lemma it is sufficient to establish that for any ϕ(x) ∈ CLγ , γ > 3, the
relation
lim
ε↓0
∫ +∞
−∞
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
ϕ(x) dx = 0
is valid. But its validity follows from the chain of inequalities,∣∣∣∣∣
∫ +∞
−∞
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
ϕ(x) dx
∣∣∣∣∣
2
6
∫ +∞
−∞
sin4 ε4(x− x
′) sin2 ε2(x− x
′)
ε4|x− x′|α
dx
∫ +∞
−∞
|x− x′|α+2|ϕ2(x)|
|x− z|4|x′ − z|4
dx
= εα−5
∫ +∞
−∞
sin4 t
4 sin2 t
2
|t|α
dt
∫ +∞
−∞
|x− x′|α+2|ϕ2(x)|
|x− z|4|x′ − z|4
dx
6 εα−5 sup
x∈R
[
|x− x′|α+2
|x− z|4|x′ − z|4(1 + |x|)γ
] ∫ +∞
−∞
sin4 t
4 sin2 t
2
|t|α
dt
∫ +∞
−∞
|ϕ2(x)|(1 + |x|)γ dx,
5 < α < min{7, γ + 2},
derived with the help of the Bunyakovskii inequality. Lemma 3.9 is proved. �
Lemma 3.10. For any γ > 3, x′ ∈ R and z ∈ C, Im z 6= 0 the relation takes place,
lim′γ
ε↓0
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
= 0.
Proof. It is true that
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
∈ L2(R; (1 + |x|)−γ) ⊂ CLγ , γ > 3.
Thus, to prove the lemma it is sufficient to establish that for any ϕ(x) ∈ CLγ , γ > 3, the
relation
lim
ε↓0
∫ +∞
−∞
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
ϕ(x) dx = 0
10 A.V. Sokolov
is valid. But its validity follows from the chain of inequalities,∣∣∣∣∫ +∞
−∞
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
ϕ(x) dx
∣∣∣∣2
6
∫ +∞
−∞
[ε(x− x′)− 2 sin ε
2(x− x
′)]4
ε6|x− x′|α
dx
∫ +∞
−∞
|x− x′|α|ϕ2(x)|
|x− z|4|x′ − z|4
dx
= εα−7
∫ +∞
−∞
[t− 2 sin t
2 ]
4
|t|α
dt
∫ +∞
−∞
|x− x′|α|ϕ2(x)|
|x− z|4|x′ − z|4
dx
6 εα−7 sup
x∈R
[
|x− x′|α
|x− z|4|x′ − z|4(1 + |x|)γ
] ∫ +∞
−∞
[t− 2 sin t
2 ]
4
|t|α
dt
∫ +∞
−∞
|ϕ2(x)|(1 + |x|)γ dx,
7 < α < min{13, γ + 4},
derived with the help of the Bunyakovskii inequality. Lemma 3.10 is proved. �
Remark 3.4. Let us consider the functionals
lim′′γ
ε↓0
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
, lim′′γ
ε↓0
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
, (3.11)
each of which is defined by a related expression in the set
lim
ε↓0
∫ +∞
−∞
(x− x′) sin2 ε4(x− x
′) sin ε
2(x− x
′)
ε2(x− z)2(x′ − z)2
ϕ(x) dx,
lim
ε↓0
∫ +∞
−∞
[ε(x− x′)− 2 sin ε
2(x− x
′)]2
ε3(x− z)2(x′ − z)2
ϕ(x) dx, (3.12)
for all test functions ϕ(x) ∈ CLγ , γ ∈ R, for which the limit from (3.12) corresponding to (3.11)
exists. It follows from Lemmas 3.9 and 3.10 that these functionals are trivial (equal to zero)
for any γ > 3, but at the same time in view of the formulae (2.39) and (2.40) from [1] these
functionals are nontrivial (different from zero) for any γ < 3. By virtue of Lemmas 3.9 and 3.10
the restrictions of the functionals (3.11) on the standard space D(R) ⊂ CLγ , γ ∈ R are equal
to zero. Hence, the supports of these functionals for any γ ∈ R do not contain any finite real
number. On the other hand, one can represent a test function ϕ(x) ∈ CLγ , γ ∈ R for any R > 0
as a sum of two functions from CLγ in the form
ϕ(x) = η(|x| −R)ϕ(x) + [1− η(|x| −R)]ϕ(x), R > 0, (3.13)
where η(x) ∈ C∞R , η(x) ≡ 1 for any x < 0, η(x) ∈ [0, 1] for any x ∈ [0, 1] and η(x) ≡ 0 for
any x > 1. In view of Lemmas 3.9 and 3.10 the values of the functionals (3.11) for ϕ(x) are
equal to their values for the second term of (3.13) for any arbitrarily large R > 0. Hence, the
values of the functionals (3.11) for a test function depend only on the behavior of this function
in any arbitrarily close (in the conformal sense) vicinity of the infinity and are independent
of values of the function in any finite interval of real axis. In this sense the supports of the
functionals (3.11) for any γ < 3 consist of the unique element which is the infinity. At last,
since (i) for any ϕ(x) ∈ CLγ and γ ∈ R the relation
limγ
R→+∞
η(|x| −R)ϕ(x) = ϕ(x)
holds; (ii) the restrictions of the functionals (3.11) on D(R) are zero for any γ ∈ R and (iii) the
functionals (3.11) are nontrivial for any γ < 3, so the latter functionals are discontinuous for
any γ < 3.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 11
4 Proofs of resolutions of identity for the model Hamiltonian
with exceptional point inside of continuous spectrum
Proof of the initial resolution of identity (3.7) of Part I is based on the following Lemmas 4.1–4.3.
Lemma 4.1. Suppose that
(1) the functions ψ(x; k), ψ0(x) and ψ1(x) are defined by the formulas (3.1) and (3.2) of Part I
for fixed α > 0, z ∈ C, Im z 6= 0 and any x ∈ R, k ∈ C, k 6= ±α;
(2) L(A) is an integration path in complex k plane, obtained from the segment [−A,A], A > α
by its simultaneous deformation near the points k = −α and k = α upwards or downwards
and the direction of L(A) is specified from −A to A.
Then for any x, x′ ∈ R and A > α the following relation is valid,∫
L(A)
ψ(x; k)ψ(x′;−k) dk
=
sinA(x− x′)
π(x− x′)
− 1
2πα
{
cos[(A+ α)(x− x′)]
A+ α
+
cos[(A− α)(x− x′)]
A− α
}
ψ0(x)ψ0(x
′)
− 1
π
[∫ A+α
A−α
cos t(x− x′)dt
t
]
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]. (4.1)
Proof. With the help of (3.1) and (3.2) of Part I and certain identical transformations one can
rearrange the left-hand part of (4.1) to the form,∫
L(A)
ψ(x; k)ψ(x′;−k) dk =
1
2π
∫
L(A)
eik(x−x
′) dk
− 1
4πα
ψ0(x)ψ0(x
′)
[∫
L(A)
∂
∂k
(
ei(k−α)(x−x
′)
k − α
)
dk +
∫
L(A)
∂
∂k
(
ei(k+α)(x−x
′)
k + α
)
dk
]
+
1
2π
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
[∫
L(A)
ei(k−α)(x−x
′)
k − α
dk −
∫
L(A)
ei(k+α)(x−x
′)
k + α
dk
]
,
where from the equality (4.1) follows trivially. Lemma 4.1 is proved. �
Lemma 4.2. In the conditions of Lemma 4.1 for any x′ ∈ R and γ > −1 the following relation
holds,
lim′γ
k→±∞
[
eik(x−x
′)
k
ψ0(x)
]
= 0.
Proof of Lemma 4.2 in view of (3.2) of Part I is quite similar to the proof of a more complicated
Lemma 3.2 from Section 3.1.
Lemma 4.3. In the conditions of Lemma 4.1 for any x′ ∈ R and γ > −1 the following relation
takes place,
lim′γ
A→+∞
{[∫ A+α
A−α
cos t(x− x′) dt
t
]
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
}
= 0.
12 A.V. Sokolov
Proof. Let us use the estimate (B12) from [4],∣∣∣∣∫ A+α
A−α
cos t(x− x′) dt
t
∣∣∣∣ 6 AC
[1 + (A− α)|x− x′|](A− α)
,
x, x′ ∈ R, A > α, C = 2 sup
ξ>0
∣∣∣∣(1 + ξ)
sin ξ
ξ
∣∣∣∣ .
Therefrom it follows that∣∣∣∣[∫ A+α
A−α
cos t(x− x′) dt
t
]
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
∣∣∣∣ 6 AD
[1 + (A− α)|x− x′|](A− α)
,
x, x′ ∈ R, A > α, D = 2C sup
x,x′∈R
|ψ0(x)ψ1(x
′)|, (4.2)
where D is a finite constant by virtue of (3.2) of Part I. The statement of Lemma 4.3 is valid in
view of the following chain of inequalities obtained with the help of (4.2) and the Bunyakovskii
inequality,∣∣∣∣∫ +∞
−∞
{[∫ A+α
A−α
cos t(x− x′) dt
t
]
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
}
ϕ(x) dx
∣∣∣∣2
6
A2D2
(A− α)2
∫ +∞
−∞
|ϕ2(x)|(1 + |x− x′|)γ dx
∫ +∞
−∞
dx
(1 + |x− x′|)γ [1 + (A− α)|x− x′|]2
=
2A2D2
(A− α)3
∫ +∞
−∞
|ϕ2(x)|(1 + |x− x′|)γ dx
∫ +∞
0
dt
[1 + t/(A− α)]γ(1 + t)2
6
2A2D2
(A− α)3
∫ +∞
−∞
|ϕ2(x)|(1 + |x− x′|)γ dx
×
∫ +∞
0
dt
(1 + t)2+γ
, −1 < γ < 0, A > α+ 1,∫ +∞
0
dt
(1 + t)2
, γ > 0
→ 0, A→ +∞,
where ϕ(x) is any function from CLγ , γ > −1. Lemma 4.3 is proved. �
Validity of the resolution of identity (3.7) of Part I in CL′γ for any γ > −1 is a corollary of
the following theorem.
Theorem 4.1. Suppose that
(1) the function ψ(x; k) is defined by the formula (3.1) of Part I for fixed α > 0, z ∈ C,
Im z 6= 0 and any x ∈ R, k ∈ C, k 6= ±α;
(2) L(A) is an integration path in complex k plane, obtained from the segment [−A,A], A > α
by its simultaneous deformation near the points k = −α and k = α upwards or downwards
and the direction of L(A) is specified from −A to A.
Then for any γ > −1 and x′ ∈ R the following relation holds,
lim′γ
A→+∞
∫
L(A)
ψ(x; k)ψ(x′;−k) dk = δ(x− x′).
Theorem 4.1 follows from Lemmas 4.1–4.3 and from the case n = 0 of Lemma 3.6.
Proof of the resolution of identity (3.7) of Part I for some bounded and slowly increasing test
functions is based on the following lemma.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 13
Lemma 4.4. In the conditions of Lemma 4.1 for any x ∈ R, x′ ∈ R and A > α the inequalities
take place,∣∣∣∣∫ A+α
A−α
cos t(x− x′)dt
t
−
{
sin[(A+ α)(x− x′)]
(A+ α)(x− x′)
− sin[(A− α)(x− x′)]
(A− α)(x− x′)
}∣∣∣∣
6
6
(A− α)2(x− x′)2
, (4.3)
|ψ0(x)| 6
(2α)3/2
| sin 2αx+ 2α(x− z)|
, (4.4)∣∣∣∣ψ0(x)−
√
2α
cosαx
x− z
∣∣∣∣ 6 √
2α
|x− z|| sin 2αx+ 2α(x− z)|
(4.5)
and ∣∣∣∣ψ1(x)−
1√
2α
sinαx
∣∣∣∣ 6 1√
2α| sin 2αx+ 2α(x− z)|
. (4.6)
Proof. The inequality (4.3) can be derived with the help of integration by parts,∣∣∣∣∫ A+α
A−α
cos t(x− x′)dt
t
−
{
sin[(A+ α)(x− x′)]
(A+ α)(x− x′)
− sin[(A− α)(x− x′)]
(A− α)(x− x′)
}∣∣∣∣
=
∣∣∣∣∫ A+α
A−α
sin t(x− x′)
x− x′
dt
t2
∣∣∣∣ = ∣∣∣∣2 ∫ A+α
A−α
1
t2
d
sin2[t(x− x′)/2]
(x− x′)2
∣∣∣∣
=
∣∣∣∣2{sin2[(A+ α)(x− x′)/2]
(A+ α)2(x− x′)2
− sin2[(A− α)(x− x′)/2]
(A− α)2(x− x′)2
}
+ 4
∫ A+α
A−α
sin2[t(x− x′)/2]
(x− x′)2
dt
t3
∣∣∣∣
6
4
(A− α)2(x− x′)2
+
4
(x− x′)2
∫ A+α
A−α
dt
t3
6
6
(A− α)2(x− x′)2
.
The inequality (4.4) follows trivially from (3.2) of Part I. The inequalities (4.5) and (4.6) can
be obtained with the help of (3.2) of Part I,∣∣∣∣ψ0(x)−
√
2α
cosαx
x− z
∣∣∣∣ = √
2α| sin 2αx cosαx|
|(x− z)[sin 2αx+ 2α(x− z)]|
6
√
2α
|x− z| | sin 2αx+ 2α(x− z)|
,∣∣∣∣ψ1(x)−
1√
2α
sinαx
∣∣∣∣ = | cos 2αx cosαx|√
2α| sin 2αx+ 2α(x− z)|
6
1√
2α| sin 2αx+ 2α(x− z)|
.
Lemma 4.4 is proved. �
The applicability of the resolution of identity (3.7) of Part I for some bounded and slowly
increasing test functions is based on the next theorem.
Theorem 4.2. Suppose that
(1) the function ψ(x; k) is defined by the formula (3.1) of Part I for fixed α > 0, z ∈ C,
Im z 6= 0 and any x ∈ R, k ∈ C, k 6= ±α;
(2) L(A) is an integration path in complex k plane, obtained from the segment [−A,A], A > α
by its simultaneous deformation near the points k = −α and k = α upwards or downwards
and the direction of L(A) is specified from −A to A;
(3) the function η(x) ∈ C∞R , η(x) ≡ 0 for any x 6 1, η(x) ∈ [0, 1] for any x ∈ [1, 2] and
η(x) ≡ 1 for any x > 2.
14 A.V. Sokolov
Then for any κ ∈ [0, 1), k0 ∈ R and x′ ∈ R the following relation holds,
lim
A→+∞
∫ +∞
−∞
[∫
L(A)
ψ(x; k)ψ(x′;−k) dk
] [
η(±x)eik0x|x|κ
]
dx = η(±x′)eik0x′ |x′|κ.
Proof of Theorem 4.2 is quite analogous to the proof of Theorem 2 from Appendix B of [4]
and it is based on the inequalities from Lemma 4.4.
Remark 4.1. Theorems 4.1 and 4.2 provide the validity of the resolution of identity (3.7)
of Part I for test functions which are linear combinations of functions η(±x)eik0x|x|κ, in gen-
eral, with different κ ∈ [0, 1) and k0 ∈ R and functions from CLγ , in general, with different
γ > −1. In particular, these theorems guarantee applicability of (3.7) of Part I to the eigen-
functions ψ(x; k) and to the associated function ψ1(x) of the Hamiltonian h (see Part I).
The resolutions of identity (3.8) and (3.9) of Part I are corollaries of the resolution of iden-
tity (3.7) of Part I and of the following Lemma 4.5.
Lemma 4.5. Suppose that
(1) the functions ψ(x; k), ψ0(x) and ψ1(x) are defined by the formulas (3.1) and (3.2) of Part I
for fixed α > 0, z ∈ C, Im z 6= 0 and any x ∈ R, k ∈ C, k 6= ±α;
(2) L±(k0; ε) with fixed k0 ∈ R and ε > 0 is an integration path in complex k plane defined by
k = k0 + ε[cos(π − ϑ)± i sin(π − ϑ)], 0 6 ϑ 6 π,
where the upper (lower) sign corresponds to the upper (lower) index of L±(k0; ε), and the
direction of L±(k0; ε) is specified from ϑ = 0 to ϑ = π.
Then for any x, x′ ∈ R and ε ∈ (0, α) the following relation is valid,(∫
L±(−α;ε)
+
∫
L±(α;ε)
)
ψ(x; k)ψ(x′;−k) dk
=
2
π
cosα(x− x′)sin ε(x− x
′)
x− x′
− 1
πα
ψ0(x)ψ0(x
′)
{
1
ε
[
1− 2 sin2
ε
2
(x− x′)
]
− ε
4α2 − ε2
cos 2α(x− x′) cos ε(x− x′)− 2α
4α2 − ε2
sin 2α(x− x′) sin ε(x− x′)
}
− 1
π
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
∫ 2α+ε
2α−ε
cos t(x− x′)dt
t
. (4.7)
(4.7) follows trivially from the same representation of the integrand ψ(x; k)ψ(x′;−k) as in
the proof of Lemma 4.1.
Proof of the resolution of identity (3.10) of Part I is based on the following Lemmas 4.6
and 4.7.
Lemma 4.6. In the conditions of Lemma 4.5 for any x′ ∈ R and γ > −1 the following relation
takes place,
lim′γ
ε↓0
{
ψ0(x)
[
ε cos 2α(x− x′) cos ε(x− x′) + 2α sin 2α(x− x′) sin ε(x− x′)
]}
= 0.
Resolutions of Identity for Some Non-Hermitian Hamiltonians. II 15
Proof. The fact that for any γ > −1 the relation
lim′γ
ε↓0
{
ψ0(x)
[
2α sin 2α(x− x′) sin ε(x− x′)
]}
= 0
holds follows from Lemma 3.7 in view of (3.2) of Part I. Hence, to prove Lemma 4.6, it is
sufficient to show that for any γ > −1 the relation
lim′γ
ε↓0
{
ψ0(x)
[
ε cos 2α(x− x′) cos ε(x− x′)
]}
= 0 (4.8)
is valid. It is true that
ψ0(x)
[
ε cos 2α(x− x′) cos ε(x− x′)
]
∈ L2(R; (1 + |x|)−γ) ⊂ CL′γ , γ > −1.
Thus, to prove (4.8) it is sufficient to establish that for any ϕ(x) ∈ CLγ , γ > −1, the relation
lim
ε↓0
∫ +∞
−∞
ψ0(x)
[
ε cos 2α(x− x′) cos ε(x− x′)
]
ϕ(x) dx = 0
holds. But in view of (3.2) of Part I its validity follows from the chain of inequalities,∣∣∣∣∫ +∞
−∞
ψ0(x)
[
ε cos 2α(x− x′) cos ε(x− x′)
]
ϕ(x) dx
∣∣∣∣2 6 ε2(∫ +∞
−∞
|ψ0(x)ϕ(x)| dx
)2
6 ε2
∫ +∞
−∞
|ψ2
0(x)|
(1 + |x|)γ
dx
∫ +∞
−∞
|ϕ2(x)|(1 + |x|)γ dx→ 0, ε ↓ 0,
derived with the help of the Bunyakovskii inequality. Thus, Lemma 4.6 is proven. �
Lemma 4.7. In the conditions of Lemma 4.5 for any x′ ∈ R and γ > −1 the following relation
holds,
lim′γ
ε↓0
{
[ψ0(x)ψ1(x
′) + ψ1(x)ψ0(x
′)]
∫ 2α+ε
2α−ε
cos t(x− x′)dt
t
}
= 0.
Proof of Lemma 4.7 with the help of the estimate from Lemma 3 from Appendix B of [4] and
the Bunyakovskii inequality is quite analogous to the proof of Lemma 4 from Appendix B of [4].
Corollary 4.1. The resolution of identity (3.10) of Part I follows from the resolution of iden-
tity (3.8) of Part I and from Lemmas 4.6, 4.7 and 3.7.
The resolution of identity (3.11) of Part I is a corollary of the resolution of identity (3.10) of
Part I and of the following Lemma 4.8.
Lemma 4.8. In the conditions of Lemma 4.5 for any x′ ∈ R and γ > 1 the following relation
takes place,
lim′γ
ε↓0
{
ψ0(x)
[
1
ε
sin2
ε
2
(x− x′)
]}
= 0.
Proof of Lemma 4.8 is analogous to the proof of Lemma 3 from Appendix of [3].
16 A.V. Sokolov
Remark 4.2. Let us consider the functional
lim′′γ
ε↓0
[
2
πεα
sin2
ε
2
(x− x′)ψ0(x)ψ0(x
′)
]
, (4.9)
where ψ0(x) is the eigenfunction (3.2) of Part I, which is defined by the expression
lim
ε↓0
∫ +∞
−∞
[
2
πεα
sin2
ε
2
(x− x′)ψ0(x)ψ0(x
′)
]
ϕ(x) dx (4.10)
for all test functions ϕ(x) ∈ CLγ , γ ∈ R, for which the limit (4.10) exists. It follows from
Lemma 4.8 that the functional (4.9) is trivial (equal to zero) for any γ > 1, but at the same
time, in view of the formula (3.12) from [1], this functional is nontrivial (different from zero)
for any γ < 1. By virtue of Lemma 4.8 the restriction of the functional (4.9) on the standard
space D(R) ⊂ CLγ , γ ∈ R is equal to zero. Hence, the support of this functional for any γ ∈ R
does not contain any finite real number. On the other hand, one can represent any test function
ϕ(x) ∈ CLγ , γ ∈ R for any R > 0 as a sum of two functions from CLγ in the form
ϕ(x) = η(|x| −R)ϕ(x) + [1− η(|x| −R)]ϕ(x), R > 0, (4.11)
where η(x) ∈ C∞R , η(x) ≡ 1 for any x < 0, η(x) ∈ [0, 1] for any x ∈ [0, 1] and η(x) ≡ 0 for any
x > 1. In view of Lemma 4.8 the value of the functional (4.9) for ϕ(x) is equal to its value for
the second term of (4.11) for any arbitrarily large R > 0. Hence, the value of the functional (4.9)
for a test function depends only on the behavior of this function in any arbitrarily close (in the
conformal sense) vicinity of the infinity and is independent of values of the function in any finite
interval of real axis. In this sense the support of the functional (4.9) for any γ < 1 consists of
the unique element which is the infinity. At last, since (i) for any ϕ(x) ∈ CLγ and γ ∈ R the
relation
limγ
R→+∞
η(|x| −R)ϕ(x) = ϕ(x)
holds; (ii) the restriction of the functional (4.9) on D(R) is zero for any γ ∈ R and (iii) the
functional (4.9) is nontrivial for any γ < 1, so the functional (4.9) for any γ < 1 is disconti-
nuous.
Acknowledgments
This work was supported by Grant RFBR 09-01-00145-a and by the SPbSU project 11.0.64.2010.
References
[1] Andrianov A.A., Sokolov A.V., Resolutions of identity for some non-Hermitian Hamiltonians. I. Exceptional
point in continuous spectrum, SIGMA 7 (2011), 111, 19 pages, arXiv:1107.5911.
[2] Gel’fand I.M., Vilenkin N.J., Generalized functions, Vol. 4, Some applications of harmonic analysis, Aca-
demic Press, New York, 1964.
[3] Sokolov A.V., Andrianov A.A., Cannata F., Non-Hermitian quantum mechanics of non-diagonalizable
Hamiltonians: puzzles with self-orthogonal states, J. Phys. A: Math. Gen. 39 (2006), 10207–10227,
quant-ph/0602207.
[4] Andrianov A.A., Cannata F., Sokolov A.V., Spectral singularities for non-Hermitian one-dimensional Hamil-
tonians: puzzles with resolution of identity, J. Math. Phys. 51 (2010), 052104, 22 pages, arXiv:1002.0742.
http://dx.doi.org/10.3842/SIGMA.2011.111
http://arxiv.org/abs/1107.5911
http://dx.doi.org/10.1088/0305-4470/39/32/S20
http://arxiv.org/abs/quant-ph/0602207
http://dx.doi.org/10.1063/1.3422523
http://arxiv.org/abs/1002.0742
1 Introduction
2 Definition of spaces of test functions and distributions
3 Proofs of resolutions of identity for the model Hamiltonians with exceptional point of arbitrary multiplicity at the bottom of continuous spectrum
3.1 Proof of the biorthogonality relations between eigenfunctions for continuous spectrum
3.2 Proofs of the resolutions of identity
4 Proofs of resolutions of identity for the model Hamiltonian with exceptional point inside of continuous spectrum
References
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