■カタラン数(その2)
Cn=ΣCkCn-1-k (k=0〜n−1)
Cn=C0Cn-1+・・・+Cn-1C0
Ck+1/Ck=(2k+2,k+1)/(k+2)・(k+1)/(2k,k)
=2(2k+1)/(k+2)
超幾何関数で表すならば,項比は
(k+1/2)(k+1)/(k+2)・4/(k+1)
より,
F(1/2,1:2:4x)
F(1/2,1:2:x)=2/(1+(1−x)^1/2)
=2(1−(1−x)^1/2)/x
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0≦m≦nなるmについて,
ΣCkCn-1-k=1/2・Cn+(2m−n)/2n(n+1)・(2m,m)(2n−2m,n−m) (k=0〜m−1)
m=nのとき,右辺は
1/2・Cn+(2n−n)/2n(n+1)・(2n,n)
=1/2・Cn+1/2・Cn=Cn
ΣCkCn-1-k=C0Cn-1+C1Cn-2+・・・+Cn-1C0
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