Leray J. Lagrangian Analysis and Quantum Mechanics: A Mathematical Structure Related to Asymptotic Expansions and the Maslov Index

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104

II,§1,7-II,§2,0

The set

of formal distributions UR on V forms a vector space

R). Its topology is defined by the following fundamental

system of neighborhoods of 0, each depending on a bounded set B of

R) and on two numbers c > 0, N > 0:

1''(B,s,N) _ {URIIa,IB < efor r < N}.

Clearly

-o(V\ER, R) is dense in .

'(V\ER, R).

By (4.6), the isomorphism (4.9) defined by S = RR'-' E Sp2(l) extends to

an isomorphism

S:.f '(V\ER V ER., R') - .y'(V\ER U ER., R)

that is continuous and local. Similarly, by (6.6), the formal differential

operator aR extends to an endomorphism

aR:R) -

that is continuous and local and satisfies theorem 6.

The scalar product of UR and UR, a formal function and distribution

on the same V\ER, is defined by formula (5.4) of theorem 5.1 when

Supp UR n Supp UR is compact; it is invariant under Sp2(l) (part 4 of

theorem 5.1) and satisfies theorem 5.2.

§2. Lagrangian Analysis

0. Summary

Synthesizing the properties of formal functions and formal differential

operators from §1, we define and study lagrangian operators, lagrangian

functions and distributions, and their scalar products in §2. These three

notions make up the structure called lagrangian analysis.

In section 1, we define lagrangian operators and study their inverses.

In section 2, we define lagrangian functions on the covering space V

of a lagrangian manifold V; their scalar product ('

) is defined when

their supports are compact.

This makes it possible to define lagrangian functions on V in section 3;

their scalar product is defined when the intersection of their support is

II,§2,0-II,§2,1

105

compact; section 3 requires the datum of a number v° e iJO, 001; it uses

the "restriction" of v to the value v° in the expressions UR of a formal

function U.

This process of defining lagrangian functions on V has a theoretical

justification, which is the possibility of defining their scalar product (I.),

and a pragmatic justification, namely, the process of many "approximate

calculations." An example is quantum physics, where 2ni/h plays the role

of our variable v e ill, 001, h being Planck's constant; here h is taken to

be infinitely small without comparing the orders of magnitude of the

numerical values taken by the terms coming into play when h is given its

physical value; h is given this physical value after completing the calcula-

tions in which h is assumed to be infinitely small. Another historical

example is celestial mechanics; H. Poincare has elucidated how such a

process is an approximate calculation capable of predicting celestial

phenomena with extreme precision using divergent series in which

ultimately only the first terms have to be retained. V. P. Maslov wants to

obtain "asymptotics" by the same process, that is, by making approximate

calculations. In chapter III, we shall apply this process to cases in which it

is certainly not an approximate calculation, and we shall recover numerical

results that are in agreement with experimental results. Therefore, to make

this process coherent is "a problem that poses itself and not a problem that

one poses" in the sense that H. Poincare meant it.

1. Lagrangian Operators

Let Z be a symplectic space and Q an open set in Z.

Definition 1.1.

Let a° e ,y °(Q) (II,§I,defnition 6.1). Let

R= R(

1 8 1_ _(v, 1 8

)

v, x'

v ax J -

V X

be the formal differential operator associated to a° and to the 1-frame

R (II,§l,definition 6.2). The lagrangian operator associated to a° is the

collection a = {aR } of these formal differential operators aR (a° given;

R arbitrary); aR is called the expression of a in the frame R.

Theorem 2.3 will justify this terminology.

We say that a is defined on Q. We define

Supp(a) = Supp(a°)

106

II,§2,

a(v,z) = ceC Vv, Z'

then a is denoted

a=c.

Formula (2.9) will justify the following definition.

Definition 1.2.

Two lagrangian operators a and b associated to a° and

b° e F°(S2) are adjoint when

b°(v, z) = a°(v, z)

Vv, z;

equivalently, by II,§1,theorem 6,4°), a and b are adjoint when aR and

bR are adjoint for every R.

Notation.

The adjoint of a is denoted a*.

Formula (1.2) and the resulting formula (2.2) will justify the following

definition.

Definition 1.3.

The composition a° o b° of a° and b° e f °(S2) is given by

(a° o b°)(v, z) _ {(exp

2v[a_,

Oz-

)I a°(v, z) b°(v,

where ° ,

is defined by II,§1,(6.11). (1.1)

[aZ

3z'

The composition a o b of the lagrangian operators a and b is the lagrangian

operator associated to a° o b°.

By part 5 of II,§1,theorem 6,

(a o b)R = aR o bR.

(1.2)

Since composition of the aR is associative, composition of the a° and

composition of the a are associative. It is easy to verify this directly by

observing that (1.1) implies

(a° o b° o c°)(v, z)

a l a a 1

exp -

2v az' aZ'

- 2v

LaZ' az" - 2v az ' 0z"

a°(v, z)-b°(v,

z")}z.z-=. . (1.3)

II,§2,1

107

This formula easily extends to the composition of any number of elements

Clearly

Supp(a o b) c Supp(a) n Supp(b),

(1.4)

(a° o b°)(v, z) = a°(v, z) b°(v, z) mod 1, (1.5)

the right-hand side being the product in F ° of the values of a° and b°:

(a° o a°)(v, z) = Fcosh 2v rLa z l

a°(v,

z). a°(v,

z') (1.6)

J =-_

The set of lagrangian operators defined on S2 is thus a noncommutative

algebra F °(S2) over the algebra 9' of formal numbers with vanishing

phase; 3 °(S2) has an identity element.

THEOREM 1.1. (Inverse of a lagrangian operator)

The operator a has an

inverse in the algebra of lagrangian operators defined on S2 if and only if

a°(v, z) : 0 mod

Vz E S2.

(1.7)

Recall that, by definition, a°(v, z) is a formal series:

a°(v, z) = Y_ 1,

a,(z)

(1.8)

,EN t'

Condition (1.7) means

a°(z) j4 0 Vz E S2.

(1.7a)

Proof.

The equivalent conditions (1.7) and (1.7a) are necessary by (1.5).

Conversely, if these conditions hold, a right inverse a' of a is an operator

associated to an element

a'(v, ') = Y v,a'(-)

,EN

°(S2) satisfying

{(exp

- 1 f -, )a°(v, z)a'0(v, z')}Z=Z. = 1,

2v ez oz' J

that is.

108

II,§2,1

ao(z)ao(z) = 1,/

((1.10)0

a0(z)ar(z) = Cr(z), (1.10)r

where cr(z) is a linear combination of the

)'a,(z)]

such that 0 < Isl = Isl = r - t - t', t' < r. These conditions determine

a'. Therefore a has a right inverse and, similarly, a left inverse; these are

identical since the composition of elements of F °(Q) is associative. The

theorem follows.

The definition of scalar products (sections 2 and 3) will use the following.

Definition 1.4.

A partition of unity is a collection (aj} of lagrangian

operators defined on Z such that Uj Supp a3 = Z is a locally finite covering

of Z and

a, = 1 (identity operator). (1.11)

This partition is said to be finer than an open covering Uk S2k = X

of X when every Supp a3 belongs to at least one of the Stk.

THEOREM 1.2.

(Partition of unity) 1°) There exist partitions of unity

finer than a given open covering of X.

2°) These can be chosen so that the operators a3 are self-adjoint.

3°) More precisely, these partitions of unity can be chosen such that each

a, has the form

a, = b* o b;, Supp b; = Supp a3,

where b3 is a lagrangian operator and b# is its adjoint.

Proof of 1 °) and 2°).

It suffices to prove the theorem when the operators

a; are replaced by C'-functions a;°:Z -+ R that are independent of v.

It is thus a classical result.

Proof of 3°).

Given a covering Uk S2k = X, we choose Cr-functions

b°: Z -+ R such that

Y [b°(z)]2 = 1,

Suppb° c S2k for some k.

II,§2,1-II,§2,2

It follows by (1.6) that

where (3,:Z -+ R, flo = 1.

b° o bV =

reNv

Let us try to find

o I

c =

Vzr Y.,

where Yr : Z - R,

yo = 1,

reN

such that

c°oc° = Yb°ob°,

that is,

cosh1

Pr'

az aZ

"" v

r v

This condition defines Yr as a function of Pr and of the

Y(Z)J

[J)s

[()5

Y, (z)]

such that

0 < I s l - Is l = 2(r - t - t'),

0 < t < r;

thus co exists, is unique, and has an inverse by theorem 1.1. Now

bi=b*, c=c*,

Ybobi=coc;

109

therefore the

ai = (bioc-1)*o(bioc-1)

constitute a partition of unity finer than the given covering Uk S2k = X.

2. Lagrangian Functions on V

Let Z be a symplectic space provided with a 2-symplectic geometry. Let

V be a lagrangian manifold in Z and let V be its universal covering space.

Theorem 4.1 of §1 justifies the following definition.

Definition 2.1.

A lagrangian.function 0 on V is defined by the datum of

110

II,§2,2

a formal function UR on V\ER for each 2-frame R, these formal functions

satisfying

= RR'-'V,, on

V\ER U ER.

VR, R'.

UR is called the expression of U in the frame R.

The support of U, Supp U, is the subset of V defined by the condition

Supp UR = (V\ER) n Supp U

VR.

This definition makes sense since S = RR'-' is local.

The set of lagrangian functions on V and the subset of those with

compact support are vector spaces over F; they are denoted F(P) and

.FF(V). Their dimension is infinite, as is proven by the following theorem.

THEOREM 2.1.

(Existence)

Every UR. E F0(V\ ER., R') is the expression

in R' of a lagrangian function U on V with compact support.

Proof.

Let R be any 2-frame. Let

S =

RR'-' E Sp2(l),

UR = SUR. ;

UR is a formal function defined on V\ER U ER, which is identically zero

in a neighborhood of ER.. We extend its definition by making the following

convention:

UR = 0 on ER,\R n ER..

Then OR becomes a formal function defined on V \ER ; U = { UR }

clearly a lagrangian function and

Supp U = Supp UR..

II,§l,theorem 4.2 proves the following.

THEOREM 2.2. (Structure)

Let iG be the lagrangian phase of V, rl a positive

regular measure on V, and x = 1 L

E X, where i e V. We make the con-

vention [rl] "2 > 0. We give V a 2-orientation; recall that [d'x]

1112

is defined

by I,§3,corollary 3 using the Maslov index.

We have

l3r+I/2

(

URIV, yr YRr( )E

(2.1)

rEN

-d

II,§2,2

111

on V\iR, where the YRr are infinitely differentiable functions

V -+ C.

flRO is independent of R, is denoted f3o, and is called the lagrangian

amplitude.

Remark 2.1.

In (2.1), the exponent 3r + i cannot be decreased: see

III,§3,theorem 4.2, where the polynomial f, has degree 3r and the rational

function g, poles of order 3r.

II,§1,theorem 6 justifies the following definition.

Definition 2.2.

Let n be an open neighborhood of V. Let a be a lagrangian

operator on n and U a lagrangian function on V. Let R and R' be 2-frames

and S = RR'-' E Sp2(l). Then

aRUR = S(aR' UR,);

thus the formal functions aRUR form a lagrangian function, which we

shall denote a V.

In other words

THEOREM 2.3.

The algebra of lagrangian operators a, b defined on SZ

acts on the vector space f (V) [and .FO(V)] of lagrangian functions U

[with compact support] defined on V;

a(bU) = (a o b)U ;

(2.2)

Supp(aU) = Supp U n fl-' Supp a,

(2.3)

ft being the projection of V onto V c Z.

Definition 2.3 of the scalar product (I ). Let U E .FO(V) and

Let us first assume that the following condition holds:

There exists a 2-frame R such that

2.4)

Supp U n ER = 0,

Supp U' n Ex = 0'

This condition means

UR E 9'0(P \i,, R),

UR E R).

We then define

(U U') _ (UR UR) E r6 for each R satisfying (2.4). 2.5)

112

II,§2,2

The right-hand side is an asymptotic class by II,§1,definition 5.1. It is

independent of R by part 4 of II,§1,theorem 5.1.

Let U and U' no longer satisfy (2.4). Let

Y ai = 1, > aj. = 1 (2.6)

be two partitions of unity (definition 1.4) sufficiently fine (theorem 1.2)

so that for every choice of (j, j') there correspond 2-frames Ri,i, satisfying

the condition

Supp ai n ER;.;

= 0,

Supp d, n ER,., = 0;

thus

(ai U I a;. U') is defined Vj, j'.

We define

(UI U') _ Y_ (aaUIaj.(Y)E`'.

i.i'

The right-hand side is independent of the choice of partitions of unity

(2.6); indeed, if

Ibk=1,

>bk'=1

is a second choice satisfying (2.7), then

Y_ (aiUl a,, C')

i1 j,

(ai OR,,

R;.r) =

(ai o bk UR;.r

ai, o b,',, UR;.r)

i.f, k, k'

_ Y (bk U l bkXP),

k,k'

since Ri,i. can be replaced by Rk,k'

THEOREM 2.4.

(Scalar product)

1°) The scalar product (-

.) is a function

of pairs U and U' of lagrangian functions on V and V' with compact

support. It is sesquilinear over F ° and takes values in

' (asymptotic

equivalence classes).

(UJU') and (U'IU) are complex conjugate and depend only on the

II,§2,2

113

behavior of U and U' at the pairs of points of V and V' projecting onto the

same point of V n V.

If VnV'=0,then (UIU')=0.

(aUI U') = (UJa* U') if the lagrangian operators a and a* are adjoint.

2°) If V = V', which implies Vi = 0i', then

(2.9)

(UI U') E', (2.10)

the phases of (U I U') being the periods of 0 (that is, the values of i f 7[z, dz]

on the cycles y of V);

(U I U') =

z,zY

flo(z)flo(z

Y(t')]g

mod

(2.11)

where the notation is that of theorem 2.2 and z and z are the points of

Supp U and Supp U' projecting onto z in V. We have

0 '< (U I U) E'.

(2.12)

The seminorm of U,

0 U U)112 E

satisfies the triangle and Schwarz inequalities:

(U U')I

(Ui

U)112.(U'

U,)112 in W.

3°) If V and V are transverse (where V and V are given 2-orientations),

then

v1/2(U U') E '; (2.13)

C2lvl

1riJt/2 (U I U')

= I

:Evn v,

where

1/2

(z mod 1 (2.14)Y

no A no

d21z

no A no and d

21z

are the measures on Z defined at the point z c- V n V'

by II,§1,(5.11) and II,§1,(5.15) respectively,

1 and 2' are the points of Supp U and Supp U' projecting onto z,