中央2項係数を含む通常型母関数
中央2項係数を含む通常型母関数
(1)通常型母関数の部分和
\[ \sum_{k=0}^{n}\frac{1}{k+1}C\left(2k,k\right)z^{k}=\frac{1}{2z}\left\{ 1-\left(1-4z\right)^{\frac{1}{2}}\right\} -\frac{C\left(2\left(n+1\right),n+1\right)}{n+2}z^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right) \](2)通常型母関数
\[ \sum_{k=0}^{\infty}\frac{1}{k+1}C\left(2k,k\right)z^{k}=\frac{1}{2z}\left\{ 1-\left(1-4z\right)^{\frac{1}{2}}\right\} \]-
\(F\)は超幾何関数。(1)
\begin{align*} \sum_{k=0}^{n}\frac{1}{k+1}C\left(2k,k\right)z^{k} & =\sum_{k=0}^{n}\frac{\left(2k\right)!}{\left(k+1\right)!k!}z^{k}\\ & =\sum_{k=0}^{n}\frac{\left(2k\right)!!\left(2k-1\right)!!}{\left(k+1\right)!k!}z^{k}\\ & =\sum_{k=0}^{n}\frac{2^{k}k!2^{k}\left(k-\frac{1}{2}\right)!}{\left(k+1\right)!k!\Gamma\left(\frac{1}{2}\right)}z^{k}\\ & =\sum_{k=0}^{n}\frac{4^{k}Q\left(\frac{1}{2},k\right)}{\left(k+1\right)!}z^{k}\\ & =\sum_{k=0}^{n}\frac{Q\left(\frac{1}{2},k\right)}{\left(k+1\right)!}\left(4z\right)^{k}\\ & =\sum_{k=0}^{\infty}\frac{Q\left(\frac{1}{2},k\right)}{\left(k+1\right)!}\left(4z\right)^{k}-\sum_{k=n+1}^{\infty}\frac{Q\left(\frac{1}{2},k\right)}{\left(k+1\right)!}\left(4z\right)^{k}\\ & =\sum_{k=0}^{\infty}\frac{\left(-1\right)^{k}P\left(-\frac{1}{2},k\right)}{\left(k+1\right)!}\left(4z\right)^{k}-\sum_{k=n+1}^{\infty}\frac{Q\left(\frac{1}{2},k\right)}{Q\left(2,k\right)}\left(4z\right)^{k}\\ & =\sum_{k=0}^{\infty}\frac{P\left(-\frac{1}{2},k\right)}{\left(k+1\right)!}\left(-4z\right)^{k}-\sum_{k=0}^{\infty}\frac{Q\left(\frac{1}{2},n+1+k\right)}{Q\left(2,n+1+k\right)}\left(4z\right)^{n+1+k}\\ & =-\frac{1}{4z}\sum_{k=0}^{\infty}\frac{P\left(-\frac{1}{2},k\right)}{\left(k+1\right)!}\left(-4z\right)^{k+1}-\frac{Q\left(\frac{1}{2},n+1\right)}{Q\left(2,n+1\right)}\left(4z\right)^{n+1}\sum_{k=0}^{\infty}\frac{Q\left(\frac{1}{2}+n+1,k\right)}{Q\left(2+n+1,k\right)}\left(4z\right)^{k}\\ & =-\frac{1}{2z}\sum_{k=0}^{\infty}\frac{P\left(\frac{1}{2},k+1\right)}{\left(k+1\right)!}\left(-4z\right)^{k+1}-\frac{Q\left(\frac{1}{2},n+1\right)}{Q\left(2,n+1\right)}\left(4z\right)^{n+1}\sum_{k=0}^{\infty}\frac{Q\left(n+\frac{3}{2},k\right)}{Q\left(n+3,k\right)}\left(4z\right)^{k}\\ & =-\frac{1}{2z}\sum_{k=0}^{\infty}C\left(\frac{1}{2},k+1\right)\left(-4z\right)^{k+1}-\frac{\left(2n+1\right)!}{2^{2n+1}n!\left(n+2\right)!}\left(4z\right)^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right)\\ & =-\frac{1}{2z}\sum_{k=1}^{\infty}C\left(\frac{1}{2},k\right)\left(-4z\right)^{k}-2\frac{C\left(2n+1,n\right)}{n+2}z^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right)\\ & =-\frac{1}{2z}\left\{ \sum_{k=0}^{\infty}C\left(\frac{1}{2},k\right)\left(-4z\right)^{k}-C\left(\frac{1}{2},0\right)\left(-4z\right)^{0}\right\} -\frac{C\left(2\left(n+1\right),n+1\right)}{n+2}z^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right)\\ & =\frac{1}{2z}\left\{ 1-\left(1-4z\right)^{\frac{1}{2}}\right\} -\frac{C\left(2\left(n+1\right),n+1\right)}{n+2}z^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right) \end{align*}(2)
\begin{align*} \sum_{k=0}^{\infty}\frac{1}{k+1}C\left(2k,k\right)z^{k} & =\lim_{n\rightarrow\infty}\sum_{k=0}^{n}\frac{1}{k+1}C\left(2k,k\right)z^{k}\\ & =\lim_{n\rightarrow\infty}\left\{ \frac{1}{2z}\left\{ 1-\left(1-4z\right)^{\frac{1}{2}}\right\} -\frac{C\left(2\left(n+1\right),n+1\right)}{n+2}z^{n+1}F\left(1,n+\frac{3}{2};n+3;4z\right)\right\} \\ & =\frac{1}{2z}\left\{ 1-\left(1-4z\right)^{\frac{1}{2}}\right\} \end{align*}ページ情報
タイトル | 中央2項係数を含む通常型母関数 |
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2項係数が0になるとき
\[
\forall m,n\in\mathbb{Z},\left(0\leq m<n\right)\lor\left(n<0\leq m\right)\lor\left(m<n<0\right)\Leftrightarrow C\left(m,n\right)=0
\]
2項係数の1項間漸化式
\[
C(x+1,y)=\frac{x+1}{x+1-y}C(x,y)
\]
2項係数の飛び飛びの総和
\[
\sum_{k=-\infty}^{\infty}C\left(mn,mk+l\right)=\frac{1}{m}\sum_{j=0}^{m-1}\left(1+\omega_{m}^{j}\right)^{mn}\left(\omega_{m}^{j}\right)^{-l}
\]
2項係数の半分までの総和
\[
\sum_{k=0}^{n-1}C\left(2n-1,k\right)=2^{2n-2}
\]