OPERATIONAL CALCULUS FOR THE CONTINUOUS LEGENDRE TRANSFORM WITH APPLICATIONS

Internat. J. Math. & Math. Sci.

VOL. 12 NO. 2 (1989) 355-362

355

OPERATIONAL CALCULUS FOR THE CONTINUOUS LEGENDRE TRANSFORM WITH APPLICATIONS

E.Y.DEEBA

Department

of Applied MatheInatical Sciences Universityof

Houston-Downtown

Houston, Texas

77002 E.L. KOH

Department

ofMathematicsand Statistics University ofRegina

Regina, Saskatchewan, Canada,^$4S 0A2

(Received

^June 13, ^{1988 and}in revised formSeptember

1988)

ABSTRACT.Thispaper developsanoperationalcalculus for thecontinuousLegendretransform introducedand studiedby

Butzer,

Stens and Wehrens

[1].

^{It isanextension ofthe work done} by Churchill et al

[2], [31

^{for the discrete case.}

^In

particular, adifferentiation theorem and ^a convolution theorem are proved and the results are appliedto the solution ofsome boundary valueproblems.

KEY

WORDSAND PHRASES. Continuous Legendre Transform, Operational Calculus, Convo- lution, Boundary Value Problems.

1980AMS SUBJECT CLASSIFICATION CODES: 44A15, 33A30,

1. INTRODUCTION. For a given function

f

^{belonging toan} appropriate function space, the continuousLegendretransformisdefined by

1

J Px(x)f(x)dx (1)

(Tf)(A)

-

where

P(x)is

^the Legendre function and

A >_ -].

^This transform has been introduced and studiedby

Butzer,

Stens and Wehrens

[1].

The discreteanalogofthe transformill

(1)

^{has been}

studied by Churchill

[2]

and Churchill and Dolph

[3].

The object of this paperis to develop

anoperational calculusfor the transforin whichisuseful insolving paxtial differential equations whoseunderlyingdifferentialforlnisgiven by

D=xx ^{(1-x)xx (2)}

Ill section 2wepresentthebackgroundmaterialneeded in thesequel. Insection3, vederive theoperationalcalculus for

(1)

^includingaconvolutiontheorem andatable oftransforms ofsome

functions.

In

^thelastsectionweapplytheresults to solvingsomeboundary value problems.

2.

PRELIMINARIES.

Werecallbasic propertiesof the transform

(Tf)(A) (see [1])

^anditnportant coutiguousrelationsthat hold for theLegendrefunction.

The Legendrefunction

P(x)

^isgiven

_

^by

_{(-)(+) -}

P(x) F(-A, A+I;I;-,2 _(k’) ^(’

2

x)’ xe(-1, 1].

P(x)

^{satisfies the}differential Since

P_(x) P_(x),

itsufficies to consider the ce A _-5"

equation

Dy + A(A +

^{1)y 0}

where

D

is givenin

(2).

Fher,i satisfies tim relations

P (1) P ⁽¹⁾

^lira.__

^{, (1+}

)(.)

^0d

lh.__,.( + .)V(.)

Thefollowingcontiguousrelations

(see [4])

^{will beuseful}in he derivationof he calculusfor

(Tf)(A).

and

(2A + 1)xP(x) (A + l)P+(x) + ^AP_(x) ()

(1 x2)P’(x) -AxP:(x) + ^APA_(x). (4)

From

(3)

^and

(4)

^weobtainthe relation

(1 x2)P’(x) _A(A + ¹⁾ _{(PA+I(X)- P_,(x)).}

2A+l (5)

The addition formula for theLegendrefunctions

(see [4])

isgivenby

F(A

^m

+ 1)p,(cosa)p,(cosl3)cosm

7

(6)

P(o )p(o Z)

p(o

)-

r(A +

where

P"(-)

^is the associated Legendre functionand ^cosy cosacos

fl +

^sin

asinflcos7

^vith

0

_<

a, /3

_<

r, ^a

+ fl ^<

a’, 7real. Formula

(6)

^{willbeusefulin}deriving the convolution theorem.

Another usefulrelationinvolving the Legendrefunctions is sinrA sin

P()P.(-)d ()

,(A_ )(A + + )

^A#,

A++#0.

Th

Lnd

^tom

^(TI)(A)

ⁱ i ieg tfom fom

Z(-1,11

^into

^ta

^p

C0(-1,1] L(-I, 1].

^For

feL(-1, I],

^{itwshow in}

[1]

^that

(Tf)(A) 0(A-I) ^A

^and

(Tf)(A ])eC0(-1,1] ^L(-1,1].

^{Fther,it w shown that if}

^{feL(-1,1] C(-1, 1]}

^{and if}

(Tf)(A )(+),

^th

^tae

^{iiofomigiv by}

f(x) T-((Tf)(A))

⁴

(Tf)(A- )Px_(-x)AsinAdA.

3. BASIC OPERATIONALPROPERTIES FOR

(Tf)(A).

^{In this sectio, we, .el,all .’}

operationalcalculus for thecontinuous Legendre transform

(Tf)(.\)

thus exl,,u,lig ^tl,, .!cl- obtainedbyChurchill

[2]

^{andChurchill and}Dolph

[3]

for the discretecase.

the Legendre transform ofsomefunctions.

Thefirstpropertyin this directioninvolvesthe Dasgivenin

(2).

CONTINUOUS LEGENDRE TRANSFORMS 357

Theorem 3.1. Let

f

^{be a function such that}

⁽ⁱ⁾ f(JeC(-1,1]

^t3

L2(-1,1]

(ii) lim_:t,(1 x2)f(z) lim._,(1 x2)f’(x)

^0and

(iii) (Tf)(A)

^{exists. Then}

k 0,1

(T(Df))(A) -A( + 1)(Tf)(1).

P_Lg..Q_[. From

(1)

togetherwith successiveintegrationbyparts,^weobtain

Px(x)Df(x)dx (T(Df))(A)

-

-2 Px(x)-x ^{(1-x ) f(x)}

^dx

Px(x)(1 x)f’(x)-

-

^-i+

--A(A+l)f_’aP(z)f(z)dx.

The result followsfromthe facts that

Px(1)

1,

P(1) a{+), lim__,+(1 + x)Px(x)

^0and

lim__,+(1 + x)P(x) ,m

^togetherwith the hypothesis

(ii).

Ts

bic operationM property reduces ^agivendifferentiMequation whichinvolvestheoper- ator

D

into Mgebrcone orintoadifferentiM equationwithoneless independentriable.

Remk3.1.

(a)

If,inThrem3.1,

Df ^D-(Df)

^d

^f()

satisfy thesamehypotheses, then

T((Df(x)))(A) (-1)A(A + ^1)(Tf)(A),

^k 1,2,....

(b)

^Wenotethat

(9)

^cbect into theform

(rI(l- r((Ill(l ( + P(rI(l. ^(o

The second operational propertyinvolves the relationslfip between the transform of^agiven function

f

^{and thefunction}

^g(x) fl ^f(t)dt.

Theorem^3.2. If

f

^{isapiecewisecontinuous}function definedon

(-1,1)

and

g(x) if_, f(t)dt

and if

(Tf)(A)

exists,then

(Tg)(A) (T/)(A + 1)- (Tf)(A- 1) 2A+l (11)

Proof. Since

D(PA(x)) -A(A + ^1)PA(x),

^itfollows that

(Tg)(A)

-2A(A + 1 xx ⁽¹

^x

^{2) PA(x) g(x)dx}

2A(A--+ 1)(1 x)P(z)9(x)l _ ⁺ _{2A(A + 1)} ⁽¹ x2)P(z)f(x)dx.

Since

P,(1)

^and

g(1)

^are defined,

g(-1)

0 andlimx__,_,,(1

+ x)P(x)

^{,i,,,___A the first}

identically zero. Thus

(Tg)(A)

2\(A + I) ⁽¹ x)P(x)f(x)dx.

The contiguousrelation

(5)

^will then imply that

(Tg)(A)

2A(A +

^{1) 2A}

+

¹

Equivalently,

(Tg)(A) (Tf)(A + 1)- (Tf)(A- 1) 2+1

Remark3.2. Similar difference relations to that of

(11)

^{can be} obtained in the following situation.

(a)

^Ifg(x)

xf(x)

^andif

(Tf)(A)

exists, then underappropriate conditionsonf,oneobtains

(Tg)(A) (A

^/

1)(Tf)(A

^/

1)/ A(Tf)(A 1) 2+I (12)

This willfollow

Dy

applying the contiguousrelation

(3).

() u () L’,(- )()

i

(TI)()

xi,

th, . ^oi

^oiion

on

f,

^thecontiguousrelation

(5)

and Theorem3.2yields

(Tg)(A) (TI)(A + 2) 2(T/)(A) + ^{(TI)(A 2)}

(2A

^/

1) (13)

The next operational property that wewill derive involves the inverse of thedifferential op- erator D. We define the inverse of

D,

denoted by

D-’,

by

D-’(f(x)) g(x)

if and only if

D(g(x)) f(x).

^If

(Tf)(A)

^is

known,

^thenwewant to relate

T((D-Xf))(A)

^tothe transform of

f.

If,foragivenfunction

f(x), D(g(x)) f(x),

^thenonintegrating twice, weobtain

x ^{/"__ f(a)dadt}

^-I-c

forsomeconstant c. If

f(x)

^is in additionanevenfunctionon

(-1,1),

^{thenone canshowby} employingacontinuityargumentthat

lim,:._,+,(1-z2)g(z) lim,._.+t(1-z)g’(x)

^0. Theorem 3.1 will thenimply that

(Tf)(A) ^T((Dg))(A) -A(A + ^1)(Tg)(A).

Equivalently,

Thus

1

(Tf)($)

(Tg)(,)

( + 1---T((Dg))()= _{-A(A +} 1--’--’--"

T(D-’I)(A)

1

-$(A + 1)(Tf)(A).

(TI)()

We

thus This last relation implies that

D-f

^{isthe inverse Legendre}transform of-

have

Theorem.3.3.

^If

f(x)

^{issuch that}

f(x)

isevenon

(-1,1), feL(-1,1] C(-1,1], (Tf)()

existsand ^(T/)(,x)

eL(a+),

^then

D-I(f(x))=T-(-(Tf)(A))A(A + ^{1) (14)}

where theinversetransform

T

^-a isgiven by

(8).

Weshall finallydevelop^aconvolutionproperty fortheLegendretransforin. Inparticular, we willshow

CONTINUOUS LEGENDRE TRANSFORMS 359 Theorem3.4. ^If

f(x)

^and

g(x)

aregivenfunctionsforwhich

(Tf)(A)

^and

^(Tg)(A)

respectively exist, thentheirproduct

(Tf)(A)(Tg)(A)

isthe transform ofthe function

h(x) ](x),g(x)

^where

h(x)

^is givenby

h(cos v)=

-

f(cos)g(cosB)sinododO

where cos/ ^cosacost

+

sinsintcos8 with 0

<

c, ^t

<

r, a

+

^r,

<

^{r and O is real. The} variables a, ^t,astd maybe interpretedasthesidesofaspherical triangleonthe unithemisphere and istheanglebetween the sidesaandu

(see

Figure

1).

Proof. From (1),

^wehave

1 tt r

P(u)9(y)dy.

Setx cosaand y cos

B.

^Then

I(o)m (o)p(o)(o)mae,.

headditionformulafor^gleLegendrefunction

(6)

will yield uponaningegrationwih

respecg

to from0 to

1

P (cos .)d

wherecos cos cos

+

^{sin sin cos}

^(s

^figure

^1).

Figure 1 Thus

(T.f)(A)(Tg)(A) f(cosa)sina

P.(cosu)g(cosfl)sinfld-rd/3d,.

In

the spherical triangle

PQR,

^wehave

cos cosccos v

+

^{sincsinvcos8.}

Usingthis relationalongwith thesinelaw and transfornation of co-ordinates, thedouble i,tegral canbewritten as:

Hence,

fo" fo" ^{(o .)g(o 3)}

^i,,

(Tf)(A)(Tg)(A)

- P,(cosv)sinv

The expressionin thebracketis afunctionofvandwethenwrite

1 f(cosa)g(cosfl)sinoMadt?

(5)

This maybe interpretedas aconvolutionproductof

f

^andgand

(Th(cos v))(A) (Tf)(A)(Tg)(A).

This provesTheorem3.4.

Geometrically, the expression

(15)

^{isthemeanvalue of}

f(cosa)g(cos)

^{over theunit hemi-} sphere

x2+y2+z

1, z

>

O. Toseethis,^wenotethat the element surfaceareaisdS sinadadS.

This isclear ifweidentify the coordinate transformationin Figure¹by

X COS

sintsin0 sinc cos0

Thus

(15)

^reads

l

fsff(cosc,)g(cos3)dS.

h(o 1

Wewill nowevaluatetheLegendretransforn ofsome fmmtions.

1.

f(x)

constant k

k ^sinn

(Tf)(A)

k 0

2./’() P,,().

^Thby

(2.)

^whve,

o.

0,,2,...,

3.

,() log( ).

1

]_ ^{P (x) log(}

¹

^x)dx

(Tf)(A)

1+ ^{1) /_,} [

^d

^(I

^d

^{P(x) log(1 x)dx}

sinrA 1 1

/ ^P

^{x d =. d}

(o)(a+)-(+-a(a+ (1-)o(1-)a.

log(I-x)]

Observe thatD(log(1- 1. Thus

sin

A

^{sin A}

(Tf)(A) (og 2)( + (A + 1 ^A=( + ¹⁾⁼

4.

f(A) f dt.

^{Byusing and3above,weoltain}

CONTINUOUS LEGBNDRE TRANSFORMS ₃₆₁

1 sin

rA

(Tf)(A) +

( + ) ( + ^)"

5.

f(x) (1

^2tx

+ x2)-1/2 .,__otnP,(x),

-1

< <

1.

From (2)

^above

(Tf)()

^sinr

2

(- .)( + . + )

Wefiny retook that for

A equM

toa non-negativeinteger,the results ofthis sectionyield those obtned in

[2]

^and

[3].

4.

APPLICATIONS. In

this sectionwe consider mineapplications ofthe Legendretransform.

Weconsiderproblearisinginheat conductionandinpotentiM theory.

A. Heat ConductionProblem. Consideranon-homogeneousbwith extremities at x 1 mdis iulaed at these end points. Let

u(x,t)

^{be the}temperatureof the b at position^z at timet. Theonedimensiondheat equationwithprescribediNtidtemperatureisgiven by

0 ^{(’ oN(’}

u(x,O) g(x),

-l <z <1

wherek, pd c ephysical constt.s representing therm conductivity, density xd specific het rpective]y. Wesumethat the thermal conductivity kisgiven byk

(1 z),

being

re

comfit. he above equation reads

0

( ^(1-x) 0u(z,t)) ^pcOu

^a

^Ot(x’t)

(,o) ()

^-1

< < .

ff

U(A,t) T(u(x,t))(A)

^d

G(A) (Tu(x,O))(A),

then, by Theorem 3.1, ^weobtnupon the application of the transform

Thesolution isgivenby

-U(A,t) ^---A(A-I-

_pc

^1)V(A,t)

v(, o) G().

w(,t) G(),-

Now

u(x, t)

canbe obtained byeitheremploying theinversionformula

(8)

^ortheconvolution theorem.

By

employingtheinversionformula and under the assumption that

u(z, t)eC(-1,1]

^t3

L2(-1,1)

^and

V U(A ,t)LI(R+),

^oneobtains

1

_(2_{)tp(_x)AsinAd A

(,t)

⁴

G(- )

On the other hand theconvolutionproperty

(Theorem 3.4)

^willyield

u(cosu,) (cosa)f(cos)sindcdO

where _a, /, 0are as in Figure ^{1 and cos} cos a cos u

+

^{sin sinu cos0 and}

f

^is the inverse ransform ofe (a

-l.

Tha is, by

(8)

I() ’

^,(

’ ^{t-)am ,xaa.}

B. Dirichlet Problem for theUnitSphere

(see[2])

^{Consider the problem} of determining the potential

v(r, cos0)

in the interiorofaunitsphere witha prescribedpotential

f(cos0)

^{on r}

1, 0

<

0

<

^r. The Laplaceequation definingthispotentialis r r sinO

(sin vo)o

^O.

Ifz cos

O,

then the equation reduces to

r(rv)rr + ((1-x2)v)

⁰

(,) f(), - _< _< .

if

v(, )

^-,d

F()

^{d,,oteepiy thLegdet,fomof}

(, )

^d

f(),

^the,upon

applying the transformto the underlying equation,weobtain

-Tj(v(,,))-

d

^{( + 1))v(,a)}

^0,

v(,) ().

,The

solution ofthisequationisgiven by

V(r, A) cr + cr

^-+).

L

^o,d, to

pp

th i,io fo

(s)

^{w ed to h}

o(,)L(-,] C(-,]

^d

V(,)L(,+).

^ThiwiUimp that 0 d

(, ) F()

^wiUipy that

, F().

Hencethe lutionisgivenby

(, ) F()

d

v(r,)

^r

F(I- )r-IP(-)IsinIdl.

CKNOWLEDGNMEN. he work ofghe firg euflmr wpgiy supported by

rese gr

from the Uversiy ofHousgon-Downown. Thesecond authorwptiMlysupportedby NSERCof Cede under ^Grog -7184.

REFERENCES

1.

Butzer, P.L.,

R.L. Stemand M. Wehrens, Thecontinuous Legendretransform, its inverse transform and applications,Internat. J. Math and Math. Sciences, Vol.

3_z

^{No. 1}

(1980),

47-67.

2. Churclfill,

R.V.,

The operational calculus of the Legendre transform, J. Math. Phys.,

( 94),

^{6- 77.}

3. Churchill, R.V. and C.L. Dolph, Inverse transforms of product of Legemlre ttzusform., Proc. Amer. Math.

Soc., _5(1954),

^93-100.

4. Erdelyi,

A.,

et al, HigherTranscendental Function,s, Vol. 1,^hIcG,w Itill,

:.Y.,

1053.