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Oscillator scaling and exact realization

Statement

Assume the Axiom of Countable Choice (The Axiom of Countable Choice (ACω)). Fix real parameters m>0, ω>0, ℏ>0, put ℓ:=ℏ/(mω)>0, and let g0,P,(gn)n∈N be the Hermite data of Hermite recurrence in L2(R,dy), so that (gn) is a complete orthonormal family by Hermite completeness. Define

D:L2(R,dy)→L2(R,dx),(Df)(x):=ℓ−1/2f(x/ℓ),

and let A be the self-adjoint diagonal operator of Diagonal adjoint and graph-core lemma for the complete orthonormal family (gn) and the eigenvalues λn=2n+1, with the core P and spectrum {2n+1:n∈N} (Diagonal spectrum).

Then:

(a) D is a linear isometry of L2(R,dy) onto L2(R,dx) with two-sided inverse (D−1g)(y)=ℓ1/2g(ℓy); the functions ψn:=Dgn, ψn(x)=ℓ−1/2gn(x/ℓ), form a complete orthonormal family of L2(R,dx);

(b) the operator

Hosc:=ℏω2 DAD−1,D(Hosc):={f∈L2(R,dx):D−1f∈D(A)},

is self-adjoint, satisfies Hoscψn=ℏω(n+12)ψn for every n, and has

D(Hosc)={f:∑nℏ2ω2(n+12)2∣cn∣2<∞},Hoscf=∑nℏω(n+12)cnψn,

where cn:=⟨D−1f,gn⟩=⟨f,ψn⟩; its spectrum is σ(Hosc)={ℏω(n+12):n∈N}, its ground energy is ℏω/2 with eigenvector ψ0, and ⟨Hoscf,f⟩≥ℏω2∥f∥2 for every f∈D(Hosc);

(c) D(P)=span⁡{ψn} is a core for Hosc, and for every polynomial p the function f:=D(pg0) is C∞ on R with

Hoscf=−ℏ22mf′′+mω22x2f

pointwise on R;

(d) for every f∈D(Hosc) and every test function φ∈Cc∞(R),

∫R(Hoscf)(x)φ(x) dx=∫Rf(x)(−ℏ22mφ′′(x)+mω22x2φ(x))dx,

so Hosc is the self-adjoint realization of the differential expression −ℏ22md2dx2+mω22x2 in the weak sense; no identification of D(Hosc) with a separately defined maximal differential-operator domain is asserted.

All parameters enter as positive real numbers; no physical identification of m,ω,ℏ with measured quantities is made.

Facts & Assumptions

Given: real m,ω,ℏ>0; ℓ=ℏ/(mω); the complete orthonormal family (gn) in L2(R,dy); the diagonal operator A with Agn=(2n+1)gn; the map D and the vectors ψn=Dgn.

[F1]

(gn) is orthonormal and complete in L2(R,dy), hence a Hilbert basis; coefficients in expansions are ⟨f,gn⟩ and ∑n∣⟨f,gn⟩∣2=∥f∥2 (Hermite orthogonality, Hermite completeness, Complex Lp classes and Euclidean test-function conventions).

[F2]

A has domain D(A)={f:∑n(2n+1)2∣⟨f,gn⟩∣2<∞}, action Agn=(2n+1)gn, is self-adjoint with σ(A)={2n+1:n∈N}, and its algebraic eigenvector span is a core for A; the Hermite polynomials Hn have degree n with nonzero leading coefficient, so triangular elimination gives P=span⁡{gn}; also ⟨Af,f⟩≥∥f∥2 for f∈D(A) (Diagonal adjoint and graph-core lemma(b),(d),(e), Diagonal spectrum, Hermite recurrence(f)).

[F3]

(−∂y2+Y2)φ=Aφ for φ∈P; explicitly −φ′′+y2φ=2a†aφ+φ on P and Agn=(2n+1)gn (Hermite orthogonality(b), [F2]).

[F4]

For a C1 diffeomorphism T:U→V of open subsets of R and h∈L1(V), ∫Vh(y)dy=∫Uh(T(x))∣T′(x)∣dx (A C^1 diffeomorphism satisfies the change-of-variables formula for L^1 functions).

[F6]

Cc∞(R)⊆L2(R) and, for g∈Cc∞(R), g′′∈L2 and x↦x2g(x)∈L2; the integral pairing (u,φ)↦∫uφ satisfies ∣∫uφ∣≤∥u∥2∥φ∥2 (Test function space d of an open set, Complex Lp classes and Euclidean test-function conventions, [F5]).

Proof

1.1givenF4algebra

[D is a bijective isometry] Define D initially on a measurable representative u of an L2(dy) class by the representative ℓ−1/2u(x/ℓ). With T(x)=x/ℓ in [F4], ∫h(x/ℓ)dx=ℓ∫h(y)dy for every h∈L1(R). If u,v represent the same class, apply this identity to h=∣u−v∣2∈L1: then ∥ℓ−1/2(u−v)(⋅/ℓ)∥22=∥u−v∥22=0, so D is independent of the representative. Applying the same change of variables to ∣u∣2 gives ∥Du∥2=ℓ−1∫∣u(x/ℓ)∣2dx=∥u∥2; D is linear, hence a linear isometry. The formula D−1v(y):=ℓ1/2v(ℓy) is also independent of representatives by the same change of variables, is linear, and satisfies D(D−1v)=v and D−1(Du)=u as L2 classes. Thus D is a bijective linear isometry.

2.1step 1.1F1

[Transport of the orthonormal basis] By step 1.1 and the definition ψn=Dgn, for all m,n we have ⟨ψn,ψm⟩L2(dx)=∫ℓ−1gn(x/ℓ)gm(x/ℓ)‾dx=∫gn(y)gm(y)‾dy=δnm by the substitution of step 1.1 applied to gngm‾∈L1 and [F1]. If h∈L2(dx) satisfies ⟨h,ψn⟩=0 for all n, then ⟨D−1h,gn⟩=⟨h,ψn⟩=0 for all n because D is an isometry, so D−1h=0 by completeness of (gn) and h=0. Hence (ψn) is a complete orthonormal family.

3.1step 1.1step 2.1F1F2

[Hosc is the diagonal operator in the transported basis] Let B be the diagonal operator of Diagonal adjoint and graph-core lemma for the complete orthonormal family (ψn) and the eigenvalues λn′=ℏω(n+12), so D(B)={f:∑n(λn′)2∣⟨f,ψn⟩∣2<∞} and Bf=∑nλn′⟨f,ψn⟩ψn. By that lemma and Diagonal spectrum, B is self-adjoint with σ(B)={λn′}‾={ℏω(n+12)} (a closed discrete set) and ⟨Bf,f⟩≥ℏω2∥f∥2 because λn′≥ℏω2. We claim B=Hosc as operators. First, ⟨f,ψn⟩=⟨D−1f,gn⟩ by the isometry of step 1.1 applied to f=D(D−1f) and ψn=Dgn; hence D(Hosc)={f:D−1f∈D(A)}=D(B). Second, if f∈D(Hosc) and cn=⟨D−1f,gn⟩, then D−1f∈D(A) means ∑n(2n+1)2∣cn∣2<∞, and the series ∑n(2n+1)cngn converges in L2 to AD−1f by [F2]; since D is a bijective isometry it preserves limits of series, so Hoscf=ℏω2D(∑n(2n+1)cngn)=∑nℏω(n+12)cnψn=Bf, and ∑n(λn′)2∣cn∣2=ℏ2ω24∑n(2n+1)2∣cn∣2<∞. Hence B=Hosc, giving self-adjointness, the eigenvalue formulas Hoscψn=ℏω(n+12)ψn, the spectrum, the domain formula, the ground energy ℏω/2 and the lower bound.

4.1step 3.1F2F3F5algebra

[The differential expression on the core] For a polynomial p put F:=pg0∈P and f:=DF, so f(x)=ℓ−1/2p(x/ℓ)g0(x/ℓ). By the chain rule and product rule [F5], f is C∞ with f′(x)=ℓ−3/2F′(x/ℓ) and f′′(x)=ℓ−5/2F′′(x/ℓ). Since ℏ2/(2mℓ2)=ℏω/2 and mω2ℓ2/2=ℏω/2 by ℓ2=ℏ/(mω), for all x one has −ℏ22mf′′(x)+mω22x2f(x)=ℓ−1/2(−ℏ22mℓ2F′′+mω2ℓ22y2F)∣y=x/ℓ=ℏω2ℓ−1/2(−F′′+y2F)(x/ℓ). By [F3], −F′′+y2F=AF as functions on R, because the operator identity on P holds for the smooth representatives; hence the right side equals ℏω2(DAF)(x)=Hoscf(x) by step 3.1. This proves the pointwise identity. Moreover, if deg⁡p≤m, then H0,…,Hm have respective degrees 0,…,m and nonzero leading coefficients by Hermite recurrence; triangular elimination shows that they span all polynomials of degree at most m. Since gn=Hng0/2nn!, every pg0∈P is a finite linear combination of the gn, and the reverse inclusion follows from gn∈P. Thus P=span⁡{gn} and D(P)=span⁡{ψn}. Since P is a core for A by [F2], its image under the unitary D is a core for Hosc=ℏω2DAD−1: for f∈D(Hosc) with D−1f∈D(A) choose φk∈P with φk→D−1f and Aφk→AD−1f; then Dφk→f and Hosc(Dφk)=ℏω2DAφk→ℏω2DAD−1f=Hoscf.

5.1step 4.1F5F6

[Weak identity for general elements] Let f∈D(Hosc) and φ∈Cc∞(R); put Sφ:=−ℏ22mφ′′+mω22x2φ, an element of L2(R) by [F6]. By step 4.1 with the core D(P), choose fk∈D(P) with fk→f and Hoscfk→Hoscf in L2. For each k, write fk=DF with F∈P; by applying [F5] separately to the real and imaginary parts on [−R,R], and using the fact that F is a polynomial times g0, whose products with φ and their derivatives tend to 0 at ±∞, integrating by parts twice gives ∫R(−ℏ22mfk′′+mω22x2fk)φ dx=∫Rfk(−ℏ22mφ′′+mω22x2φ)dx. By step 4.1 the left side is ∫(Hoscfk)φ. Letting k→∞, [F6] and [F5] give ∣∫(Hoscfk−Hoscf)φ∣≤∥Hoscfk−Hoscf∥2∥φ∥2→0 and ∣∫(fk−f)Sφ∣≤∥fk−f∥2∥Sφ∥2→0, so the identity of the statement holds for f.

6.1step 2.1step 3.1step 4.1step 5.1∎

[Summary of the hypotheses used] The construction uses only the completeness of (gn) (step 2.1), the diagonal lemma and spectrum (step 3.1), the eigenvalue equation on P (step 4.1) and the two integration-by-parts limits (step 5.1); no claim identifies D(Hosc) with the domain of a maximal differential expression, and no physical meaning is attached to m,ω,ℏ or to f.

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