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多重超幾何級数のFourier-Legendre展開

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βn=22n(2nn),K(x)=π2n=0βn2x2n,A=n=0βn3,B=n=1(2nβn)3C=n=0βn4,D=n=1(2nβn)4
E=n=0βn4m=1n(2mβm)3,F=n=1(2nβn)4m=0nβm3,G=n=0βn4m=1n(2mβm)4,H=n=1(2nβn)4m=0nβm4
γn(1)=m=0nβm2βnm,γn(2)=m=1nβnm(2mβm)2,J=n=0βn2γn(2)

Legendre

Legendre多項式Pn(x)は次式で定義されます.

112tx+t2=n=0tnPn(x)

 
Pn(x)は次の漸化式・微分方程式を満たします.

(2n+1)xPn(x)=(n+1)Pn+1(x)+nPn1(x)(2n+1)(1x2)ddxPn(x)=n(n+1)(Pn1(x)Pn+1(x))ddx(1x2)ddxPn(x)+n(n+1)Pn(x)=0

また,これらを用いて次の等式を証明することができます.

(ddxx(1x2)ddxx)f(x)Pn(x)=ddxx(1x2)(Pn(x)ddxf(x)f(x)ddxPn(x))n3Pn1(x)+(n+1)3Pn+1(x)2n+1f(x)(1){P2n(x)f(x)=(1)nβn3(f(x)m=1n(1)m1(4m1)(2mβm)3(ddxx(1x2)(P2m1(x)ddxf(x)f(x)ddxP2m1(x))(ddxx(1x2)ddxx)f(x)P2m1(x)))P2n1(x)f(x)=(1)n1(2nβn)3m=0n1(1)m(4m+1)βk3(ddxx(1x2)(P2m(x)ddxf(x)f(x)ddxP2m(x))(ddxx(1x2)ddxx)f(x)P2m(x))

 
Fourier-Legendre展開f(x)のFourier-Legendre展開は次式で定義されます.

f(x)=12n=0(2n+1)Pn(x)11f(t)Pn(t)dt

また,本稿では,f(x)Pn(x)(0,1)で積分したものをFL係数と呼ぶことにします.nの偶奇でPn(x)の偶奇が異なり,f(x)の偶奇も分けて考えるのが妥当と思います.

multiplehypergeometricseries

次の微分方程式について考えてみます.

(ddxx(1x2)ddxx)f(x)=g(x)

f(x),g(x)を偶奇で分けて考えると,次のように表現できます.

{g(x)=n=1gnx2n1,f(x)=n=0βn2x2nm=1ngm(2mβm)2(A)g(x)=n=0gnx2n,f(x)=n=1x2n1(2nβn)2m=0n1βm2gm(B)

(1)式より,g(x)のFL係数が計算できるならば,f(x)のFL係数が計算でき,ひいては多重級数のFL係数も計算できることになると思います.
以降,上記のようなHypergeometric termの多重和のみを扱うこととします.(1)式の両辺を積分すると,

{01P2n(x)f(x)dx=(1)nβn3(01f(x)dxm=1n(1)m1(4m1)(2mβm)3(limx1[x(1x2)(P2m1(x)ddxf(x)f(x)ddxP2m1(x))]limx0[x(1x2)(P2m1(x)ddxf(x)f(x)ddxP2m1(x))]01P2m1(x)g(x)dx))01P2n1(x)f(x)dx=(1)n1(2nβn)3m=0n1(1)m(4m+1)βm3(limx1[x(1x2)(P2m(x)ddxf(x)f(x)ddxP2m(x))]limx0[x(1x2)(P2m(x)ddxf(x)f(x)ddxP2m(x))]01P2m(x)g(x)dx)

ここで,limx1x(1x2)f(x)ddxPk(x)=limx0x(1x2)Pk(x)ddxf(x)=limx0x(1x2)f(x)ddxPk(x)=0であり,

(A)limx1x(1x2)Pk(x)ddxf(x)=limx1(1x2)f(x)=2πn=1gn(2nβn)2(B)limx1x(1x2)Pk(x)ddxf(x)=limx1(1x2)f(x)=π2n=0βn2gn

となります.

(2){01P2n(x)f(x)dx=(1)nβn3(01f(x)dxm=1n(1)m1(4m1)(2mβm)3(limx1(1x2)f(x)01P2m1(x)g(x)dx))01P2n1(x)f(x)dx=(1)n1(2nβn)3m=0n1(1)m(4m+1)βm3(limx1(1x2)f(x)01P2m(x)g(x)dx)

Fourier-Legendre展開

具体的なgnについて考えていきます.

1.gn=βn

g(x)=n=0βnx2n=11x2,f(x)=n=1x2n1(2nβn)2m=0n1βm3

とすれば,

{01P2n(x)f(x)dx=(1)nβn3(01f(x)dxm=1n(1)m1(4m1)(2mβm)3(π2A1(2mβm)2l=0m1(1)l(4l+1)βl3))01P2n1(x)f(x)dx=π2(1)n1(2nβn)3m=0n1(1)m(4m+1)βm3(Aβm2)
f(x)=π2n=1(1)n1(4n1)P2n1(x)(2nβn)3m=0n1(1)m(4m+1)βm3(Aβm2)

と書けることになります.いま,右辺のmに関する和はn0に収束することよりnmと書き換えることができ,n=1x2n1(2nβn)2のFL展開を思い出すと,f^(x)=n=0βn3m=1nx2m1(2mβm)2と書くことで

f^(x)=π2n=0(1)n(4n+1)βn5m=1n(1)m1(4m1)P2m1(x)(2mβm)3

となります.

2.gn=12nβn

g(x)=n=1x2n12nβn=sin1x1x2,f(x)=n=0βn2x2nm=1n1(2mβm)3,f^(x)=n=11(2nβn)3m=0n1βm2x2m

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πB1(2mβm)2)01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(2πBβm2(π28+l=1m(1)l1(4l1)(2l)2(2l1)βl))
f(x)=n=0(1)n(4n+1)βn3P2n(x)n<m(1)m1(4m1)(2mβm)3(2πB1(2mβm)2)f^(x)=n=1(1)n1(4n1)(2nβn)5m=0n1(1)m(4m+1)βm3P2m(x)

3.gn=βn2

g(x)=n=0βn2x2n=2πK(x),f(x)=n=1x2n1(2nβn)2m=0n1βm4,f^(x)=n=0βn4m=1nx2m1(2mβm)2

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(π2C2π(1)m1(2mβm)3l=0m1(1)l(4l+1)βl3)01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(π2C2π(1)mβm3m<l(1)l1(4l1)(2lβl)3)
f(x)=n=1(1)n1(4n1)P2n1(x)(2nβn)3nm(1)m(4m+1)βm3(π2C2π(1)mβm3m<l(1)l1(4l1)(2lβl)3)f^(x)=2πn=1(1)n1(4n1)(2nβn)3m=0n1(4m+1)βm6l=1m(1)l1(4l1)P2l1(x)(2lβl)3

4.gn=1(2nβn)2

g(x)=n=1x2n1(2nβn)2,f(x)=n=0βn2x2nm=1n1(2mβm)4,f^(x)=n=11(2nβn)4m=0n1βm2x2m

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πD+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3)01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(2πD(1)mβm3m<l(1)l1(4l1)(2lβl)3(π2(1)l1βl2l1))
f(x)=n=0(1)n(4n+1)βn3P2n(x)n<m(1)m1(4m1)(2mβm)3(2πD+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3)f^(x)=π2n=0(1)n(4n+1)βn3m=1n4m1(2mβm)6l=0m1(1)l(4l+1)βl3P2l(x)

5.gn=βn2m=1n1(2mβm)3

g(x)=n=0βn2x2nm=1n1(2mβm)3,f(x)=n=1x2n1(2nβn)2m=0n1βn4l=1m1(2lβl)3,f^(x)=n=11(2nβn)3m=0n1βm4l=1mx2l1(2lβl)2

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(π2E+(1)m1(2mβm)3ml(1)l(4l+1)βl3(2πBβl2(π28+k=1l(1)k1(4k1)(2k)2(2k1)βk)))01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(π2E(1)mβm3m<l(1)l1(4l1)(2lβl)3(2πB1(2lβl)2))
f(x)=n=1(1)n1(4n1)P2n1(x)(2nβn)3nm(1)m(4m+1)βm3(π2E(1)mβm3m<l(1)l1(4l1)(2lβl)3(2πB1(2lβl)2))f^(x)=n=1(1)n1(4n1)(2nβn)5m=0n1(4m+1)βm6l=1m(1)l1(4l1)P2l1(x)(2lβl)3

6.gn=1(2nβn)2m=0n1βm3

g(x)=n=1x2n1(2nβn)2m=0n1βm3,f(x)=n=0βn2x2nm=1n1(2mβm)4l=0m1βl3,f^(x)=n=0βn3m=1n1(2mβm)4l=0m1βl2x2l

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πF+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3(Aβl2))01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(2πF(1)mβm3m<l(1)l1(4l1)(2lβl)3(π2A1(2lβl)2k=0l1(1)k(4k+1)βk3))
f(x)=n=0(1)n(4n+1)βn3P2n(x)n<m(1)m1(4m1)(2mβm)3(2πF+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3(Aβl2))f^(x)=π2n=0(1)n(4n+1)βn5m=1n4m1(2mβm)6l=0m1(1)l(4l+1)βl3P2l(x)

7.gn=βn2m=1n1(2mβm)4

g(x)=n=0βn2x2nm=1n1(2mβm)4,f(x)=n=1x2n1(2nβn)2m=0n1βn4l=1m1(2lβl)4,f^(x)=n=11(2nβn)4m=0n1βm4l=1mx2l1(2lβl)2

であり,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(π2G(1)m1(2mβm)3l=0m1(1)l(4l+1)βl3(2πD(1)lβl3l<k(1)k1(4k1)(2kβk)3(π2(1)k1βk2k1)))01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(π2G(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πD+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3))
f(x)=n=1(1)n1(4n1)P2n1(x)(2nβn)3nm(1)m(4m+1)βm3(π2G(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πD+π2(1)m1(2mβm)3ml(1)l(4l+1)βl3))f^(x)=n=0(1)n(4n+1)βn3m=1n4m1(2mβm)6l=0m1(4l+1)βl6k=1l(1)k1(4k1)P2k1(x)(2kβk)3

8.gn=1(2nβn)2m=0n1βm4

g(x)=n=1x2n1(2nβn)2m=0n1βm4,f(x)=n=0βn2x2nm=1n1(2mβm)4l=0m1βl4,f^(x)=π2n=0βn4m=1n1(2mβm)4l=0m1βl2x2l

とすれば,

{01P2n(x)f(x)dx=(1)nβn3n<m(1)m1(4m1)(2mβm)3(2πH+(1)n1(2nβn)3nm(1)m(4m+1)βm3(π2C2π(1)mβm3m<l(1)l1(4l1)(2lβl)3))01P2n1(x)f(x)dx=(1)n1(2nβn)3nm(1)m(4m+1)βm3(2πH(1)nβn3n<m(1)m1(4m1)(2mβm)3(π2C2π(1)m1(2mβm)3l=0m1(1)l(4l+1)βl3))
f(x)=n=0(1)n(4n+1)βn3P2n(x)n<m(1)m1(4m1)(2mβm)3(2πH+(1)n1(2nβn)3nm(1)m(4m+1)βm3(π2C2π(1)mβm3m<l(1)l1(4l1)(2lβl)3))f^(x)=2πn=1(1)n1(4n1)(2nβn)3m=0n1(4m+1)βm6l=1m4l1(2lβl)6k=0l1(1)k(4k+1)βk3P2k(x)

となります.
次に,x1x2とした場合のFL展開を計算します.
まず,h(x)=f(1x2),j(x)=g(1x2)とし,微分方程式は

(ddxx(1x2)ddxx)h(x)=x1x2j(x)

であり,以下の場合分けもします.

{j(x)=n=1gn(1x2)2n1,h(x)=n=0βn2(1x2)nm=1ngm(2mβm)2(C)j(x)=n=0gn(1x2)n,h(x)=n=1(1x2)2n1(2nβn)2m=0n1βm2gm(D)

また,limx1x(1x2)h(x)ddxPk(x)=limx1x(1x2)Pk(x)ddxh(x)=limx0x(1x2)h(x)ddxPk(x)=0

{(C)limx0x(1x2)Pk(x)ddxh(x)=Pk(0)limx0xh(x)=2πPk(0)n=1gn(2nβn)2(D)limx0x(1x2)Pk(x)ddxh(x)=Pk(0)limx0xh(x)=π2Pk(0)n=0βn2gn

であるので,

{01P2n(x)h(x)dx=(1)nβn3(01h(x)dx+m=1n(1)m1(4m1)(2mβm)301xP2m1(x)j(x)1x2dx)01P2n1(x)h(x)dx=(1)n1(2nβn)3m=0n1(1)m(4m+1)βm3((1)mβmlimx0xh(x)01xP2m(x)j(x)1x2dx)

となります.01xPn(x)j(x)1x2dxをうまく計算するために制限がかかりそうです.

1.gn=βn

j(x)=n=0βn(1x2)n=1x,h(x)=n=1(1x2)2n1(2nβn)2m=0n1βm3

とすれば,

{01P2n(x)h(x)dx=(1)nβn3(01h(x)dx+m=1n(1)m1(4m1)(2mβm)5l=0m1(1)l(4l+1)βl3)01P2n1(x)h(x)dx=π2(1)n1(2nβn)3m=0n1(4m+1)βm4(A(1)mβm)
h(x)=n=0(1)n(4n+1)βn3P2n(x)(B+m=1n(1)m1(4m1)(2mβm)5l=0m1(1)l(4l+1)βl3)

2.gn=12nβn

j(x)=n=1(1x2)2n12nβn=1x(π2sin1x),h(x)=n=0βn2(1x2)nm=1n1(2mβm)3,h^(x)=n=11(2nβn)3m=0n1βm2(1x2)m

とすれば,

{01P2n(x)h(x)dx=(1)nβn3(01h(x)dx+π2m=1n(1)m1(4m1)(2mβm)5ml(1)l(4l+1)βl3)01P2n1(x)h(x)dx=(1)n1(2nβn)3m=0n1(4m+1)βm4(2πB(1)mβmm<l(1)m1(4m1)(2m)2(2m1)βm)01P2n1(x)h^(x)dx=(1)n1(2nβn)3m=0n1(1)m(4m+1)βm5m<l(1)m1(4m1)(2m)2(2m1)βm
h(x)=n=0(1)n(4n+1)βn3P2n(x)(01h(x)dx+π2m=1n(1)m1(4m1)(2mβm)5ml(1)l(4l+1)βl3)

3.gn=γn1(1)

j(x)=n=1γn1(1)(1x2)2n1=1x2xn=0βn2(1x2)n,h(x)=n=0βn2(1x2)nm=1nγm1(1)(2mβm)2,h^(x)=n=1γn1(1)(2nβn)2m=0n1βm2(1x2)m

とすれば,

{01P2n(x)h(x)dx=(1)nβn3(01h(x)dx+2πm=1n4m1(2mβm)6l=0m1(4l+1)βl4)01P2n1(x)h(x)dx=π2(1)n1(2nβn)3m=0n1(4m+1)βm4(Cβm2)01P2n1(x)h^(x)dx=π2(1)n1(2nβn)3m=0n1(4m+1)βm6
h(x)=n=0(1)n(4n+1)βn3P2n(x)(01h(x)dx+2πm=1n4m1(2mβm)6l=0m1(4l+1)βl4)

4.gn=γn(2)

j(x)=n=0γn(2)(1x2)n=1x2xn=1(1x2)2n1(2nβn)2,h(x)=n=1(1x2)2n1(2nβn)2m=0n1βm2γm(2)

{01P2n(x)h(x)dx=(1)nβn3(01h(x)dx+m=1n4m1(2mβm)6(π2m=0n1(4m+1)βm4m=0n1(1)mβm2m+1l=0m(1)l(4l+1)βl3+m=1n1(1)m1βm2ml=0m1(1)l(4l+1)βl3))01P2n1(x)h(x)dx=π2(1)n1(2nβn)3m=0n1(4m+1)βm4(Jβm2(π28+l=1m(1)l1(4l1)(2l)2(2l1)βl))

となります.
ここまでをまとめると

f(x)FL展開
n=0βnx2nπ2n=0(4n+1)βn2P2n(x)
n=1x2n12nβnn=14n1(2nβn)2P2n1(x)
n=0βn2x2n2πn=1(1)n1(4n1)(2nβn)3m=0n1(1)m(4m+1)βm3P2m(x)
n=1x2n1(2nβn)2π2n=0(1)n(4n+1)βn3m=1n(1)m1(4m1)(2mβm)3P2m1(x)
n=0βn3m=1nx2m1(2mβm)2π2n=0(1)n(4n+1)βn5m=1n(1)m1(4m1)P2m1(x)(2mβm)3
n=11(2nβn)3m=0n1βm2x2mn=1(1)n1(4n1)(2nβn)5m=0n1(1)m(4m+1)βm3P2m(x)
n=0βn4m=1nx2m1(2mβm)22πn=1(1)n1(4n1)(2nβn)3m=0n1(4m+1)βm6l=1m(1)l1(4l1)P2l1(x)(2lβl)3
n=11(2nβn)4m=0n1βm2x2mπ2n=0(1)n(4n+1)βn3m=1n4m1(2mβm)6l=0m1(1)l(4l+1)βl3P2l(x)
n=11(2nβn)3m=0n1βm4l=1mx2l1(2lβl)2n=1(1)n1(4n1)(2nβn)5m=0n1(4m+1)βm6l=1m(1)l1(4l1)P2l1(x)(2lβl)3
n=0βn3m=1n1(2mβm)4l=0m1βl2x2lπ2n=0(1)n(4n+1)βn5m=1n4m1(2mβm)6l=0m1(1)l(4l+1)βl3P2l(x)
n=11(2nβn)4m=0n1βm4l=1mx2l1(2lβl)2π2n=0(1)n(4n+1)βn3m=1n4m1(2mβm)6l=0m1(4l+1)βl6k=1l(1)k1(4k1)P2k1(x)(2kβk)3
n=0βn4m=1n1(2mβm)4l=0m1βl2x2l2πn=1(1)n1(4n1)(2nβn)3m=0n1(4m+1)βm6l=1m4l1(2lβl)6k=0l1(1)k(4k+1)βk3P2k(x)
n=0βn2(1x2)nπ2n=0(1)n(4n+1)βn3P2n(x)
n=1(1x2)2n1(2nβn)2m=0n1βm3n=0(1)n(4n+1)βn3P2n(x)(B+m=1n(1)m1(4m1)(2mβm)5l=0m1(1)l(4l+1)βl3)
n=0βn2(1x2)nm=1n1(2mβm)3n=0(1)n(4n+1)βn3P2n(x)(01f(x)dx+π2m=1n(1)m1(4m1)(2mβm)5ml(1)l(4l+1)βl3)
n=0βn2(1x2)nm=1nγm1(1)(2mβm)2n=0(1)n(4n+1)βn3P2n(x)(01f(x)dx+2πm=1n4m1(2mβm)6l=0m1(4l+1)βl4)

となります.

4乗のFourier-Legendre展開

(1)式を受けて,4乗となる式もどうなるか気になりましたので計算してみました.

(2n+1)xPn(x)=(n+1)Pn+1(x)+nPn1(x)[1](2n+1)(1x2)ddxPn(x)=n(n+1)(Pn1(x)Pn+1(x))[2]ddx(1x2)ddxPn(x)=n(n+1)Pn(x)[3]

[2],[3]式より

(2n+1)(1x2)d2dx2(1x2)ddxPn(x)=(n4+2n3+n2)Pn1(x)+(n4+2n3+n2)Pn+1(x)

[1],[3]式より

(2n+1)xddx(1x2)ddxPn(x)=(n3+n2)Pn1(x)(n3+2n2+n)Pn+1(x)

以上の式を適当に足し引きして整理すれば

(ddx(1x2)ddx(1x2)ddx(1x2)ddx+x)Pn(x)=12n+1((n+1)4Pn+1(x)n4Pn1(x))

となりました.また,これにより

{01f(x)P2n1(x)dx=1(2nβn)4m=0n1(4m+1)βm4(limx1D2m(x)limx0D2m(x)01g(x)P2m(x)dx)01f(x)P2n(x)dx=βn4(01f(x)dx+m=1n4m1(2mβm)4(limx1D2m1(x)limx0D2m1(x)01g(x)P2m1(x)dx))

Dn(x):=(1x2)ddx(1x2)ddxPn(x)f(x)(1x2)ddxPn(x)(1x2)ddxf(x)+Pn(x)(1x2)ddx(1x2)ddxf(x)(1x2)Pn(x)f(x)

g(x):=(ddx(1x2)ddx(1x2)ddx(1x2)ddx+x)f(x)[]

となりました.また,[]式の解として

f(x)=2π20x(K(1x2)K(1+t2)K(1+x2)K(1t2))2g(t)dt

があることがわかりました.これをみると

limx1Dn(x)=4π201K(1t2)2g(t)dt,limx0Dn(x)=0

となりそうです.f(x)は奇関数なので

f(x)=n=14n1(2nβn)4P2n1(x)m=0n1(4m+1)βm4(4π201K(1t2)2g(t)dt01g(t)P2m(t)dt)

となります.

f¯(x)=sign(x)f(x)

ところで,f¯(x)=sign(x)f(x)と定義するならば,f(x)f¯(x)の偶奇は逆になり,また0<x<1において両者は一致することにより

f(x)=n=0(4n+1)P2n(x)01f(t)P2n(t)dt=n=1(4n1)P2n1(x)01f(t)P2n1(t)dt=f¯(x)(0<x<1)

と書けるのではないでしょうか.


投稿日:2024923
更新日:2024109
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読み込み中...
読み込み中
  1. Legendre
  2. multiplehypergeometricseries
  3. Fourier-Legendre展開
  4. 4乗のFourier-Legendre展開
  5. f¯(x)=sign(x)f(x)