■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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