[PDF] 1. Préliminaires La convergence de lintégrale impropre ? +? dt est





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AGRÉGATIONINTERNEDEMATHÉMATIQUES

L'INTÉGRALEDEDIRICHLETZ

+1

0sin(t)tdt.

PATRICELASSÈRE

0sin(t)

tdt.

1.Préliminaires

Laconvergencedel'intégraleimpropreR+1

0sin(t)

tdtestclassique:iln'yapasdeproblèmes intégrationparpartiesdonne Z x

1sin(t)

tdt= cos(t)t x 1Z x

0cos(t)

t2dt jcos(t)=t2jt22L1([1;+1[))versR1

1cos(t)=t2dt2R.ParconséquentR+1

0sin(t)

tdtconverge.

Parcontrel'intégraleR+1

0jsin(t)

àécrirepourtoutentierN2

Z N 1 sint t dtNX k=1Z k+3=4 k+=4 sint t dt NX k=1p2 2Z k+3=4 k+=4dtt NX k=1 p 2 k+3=4p2NX k=11k%+1:

0jsin(t)

tjdt;2]0;1]...). 1

2PATRICELASSÈRE

Onpeutaussiplussimplementécrire

sint t sin2t t =1cos2t

2t:=g(t)0

tjenl'inni. Z +1 0 sin(t) t 2 dt= sin2tt +Z +1

02sin(t)cos(t)tdt=Z

+1

0sin(2t)tdt

d'oùlaremarquable1formule Z +1

0sin(t)

tdt=Z +1 0 sin(t)t 2 dt:

2.CalculsdeZ

+1

0sin(t)

tdt

Exercice1:SoitF(x)=Z

+1

0sin(xt)t(t2+1)dt.

1)PréciserledomainededénitiondeF.

3)ExprimerF(x)enfonctiondeC:=Z

+1

0sin(t)

tdt

4)EndéduirelavaleurdeC.

t(t2+1).

êSoita>0,pourx2[a;a]ett2R+ona

jf(x;t)j= sin(xt) t1t2+1 jxj t2+1at2+12L1(R+); C

1OnaaussiR+1

0sin(t)

tdt=P n1sin(n)n=2 +1

0sin(t)tdt.3

ê@xf(x;t)=cos(xt)

t2+1,onadoncpourtoutx2Rett2R+ j@xf(x;t)j= cos(xt) t2+1 1 t2+12L(R); +1

0cos(xt)

t2+1dt: @2xf(x;t) tsin(xt) t2+1 t!1 sin(xt) t etcettedernièren'est(comme sin(t)t l'intégrale:soitx2R?: F

0(x)=Z

+1

0cos(xt)

t2+1dt=sin(xt)x(t2+1) 1 0+Z +1

02tsin(xt)

x(t2+1)2dt Z +1

02tsin(xt)

x(t2+1)2dt:

Ainsi,pourx6=0ona

(8)F0(x)=Z +1

0cos(xt)

t2+1dt=Z +1

02tsin(xt)x(t2+1)2dt

paramètres,eneetsoita>0,pourxa @x

2txsin(xt)(t2+1)2

2t x2sin(xt)(t2+1)2

2t2cos(xt)

x(t2+1)2 j2tja2(t2+1)2+2t2a(t2+1)22L1(R) F

00(x)=Z

+1 0 2t x2sin(xt)(t2+1)2+2t2cos(xt)x(t2+1)2 dt;8x2R?:

4PATRICELASSÈRE

F

00(x)=Z

+1 0 2t (t2+1)2sin(xt)x2+2t2cos(xt)x(t2+1)2 dt sin(xt) x2(t2+1)tcos(xt)x(t2+1) 1 0Z +1 0 xcos(xt) x2(t2+1)cos(xt)xtsin(xt)x(t2+1) dt =Z +1

0tsin(xt)

t2+1dt: F

00(x)=Z

+1 0@ 2 @x2f(x;t)dt;x2R?; F

0(x)=Z

+1

0cos(xt)

t2+1dt;8x2R; F

00(x)=Z

+1

0tsin(xt)

t2+1dt;8x2R?:

êAinsi,pourtoutx2R?,

F(x)F00(x)=Z

+1

0sin(xt)

t(t2+1)dt+Z +1

0tsin(xt)x(t2+1)dt

Z +1

0sin(xt)

tdt=(

C;8x2R?

C;8x2R?

+etFF00=CsurR? ce quinousdonne

F(x)=(

ae x+bex+C;8x2R? ce x+dexC;8x2R?

Fétantimpaire,a=d;b=csoit

F(x)=(

ae x+bex+C;8x2R? bexaexC;8x2R? +1

0sin(t)tdt.5

F(0)=0=limx!0+F(x)=a+b+C=limx!0F(x)=abC

2a==2C;2b=C=2etnalement

(4)F(x)=

2sh(x)Cch(x)+C;8x2R?

F(x)C=F00(x)=Z

+1 0 2t x2sin(xt)(t2+1)2+2t2cos(xt)x(t2+1)2 dt onaladomination 2t x2sin(xt)(t2+1)2+2t2cos(xt)x(t2+1)2 2t a2(t2+1)2+2t2a(t2+1)22L1(R+): lim x!+1(F(x)C)=Z +1 0 limx!+1 2t x2sin(xt)(t2+1)2+2t2cos(xt)x(t2+1)2 dt=0 soitavec(4) lim x!+1F(x)=CetF(x)+1 2C e x+C quidonnent C=Z +1

0sin(t)

tdt=2:

C.Q.F.D.o

0sin(t)textdt.

1)Montrerquef2C1(R?

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