パスカルの法則の応用
パスカルの法則の応用
\(n\in\mathbb{N}_{0}\)とする。
\(n\in\mathbb{N}_{0}\)とする。
(1)
\[ C\left(x+n,y+n\right)=C\left(x,y+n\right)+\sum_{k=0}^{n-1}C\left(x+k,y+n-1\right) \](2)
\[ C\left(x+n,y+n\right)=C\left(x,y\right)+\sum_{k=0}^{n-1}C\left(x+k,y+k+1\right) \](3)
\[ C\left(x+n,y+n\right)=\left(-1\right)^{n}\left\{ C\left(x+n,y\right)-\sum_{k=0}^{n-1}\left(-1\right)^{k}C\left(x+n+1,y+1+k\right)\right\} \](1)
\begin{align*} C\left(x+n,y+n\right) & =C\left(x+n-1,y+n\right)+C\left(x+n-1,y+n-1\right)\\ & =C\left(x,y+n\right)+\sum_{k=0}^{n-1}\left\{ C\left(x+n-k,y+n\right)-C\left(x+n-\left(k+1\right),y+n\right)\right\} \\ & =C\left(x,y+n\right)+\sum_{k=0}^{n-1}C\left(x+n-\left(k+1\right),y+n-1\right)\\ & =C\left(x,y+n\right)+\sum_{k=1}^{n}C\left(x+n-k,y+n-1\right)\\ & =C\left(x,y+n\right)+\sum_{k=0}^{n-1}C\left(x+k,y+n-1\right) \end{align*}(2)
\begin{align*} C\left(x+n,y+n\right) & =C\left(x+n-1,y+n\right)+C\left(x+n-1,y+n-1\right)\\ & =C\left(x,y\right)+\sum_{k=0}^{n-1}\left\{ C\left(x+n-k,y+n-k\right)-C\left(x+n-\left(k+1\right),y+n-\left(k+1\right)\right)\right\} \\ & =C\left(x,y\right)+\sum_{k=0}^{n-1}C\left(x+n-\left(k+1\right),y+n-k\right)\\ & =C\left(x,y\right)+\sum_{k=0}^{n-1}C\left(x+k,y+k+1\right) \end{align*}(3)
\begin{align*} C\left(x+n,y+n\right) & =-C\left(x+n,y+n-1\right)+C\left(x+n+1,y+n\right)\\ & =\left(-1\right)^{n}C\left(x+n,y\right)-\sum_{k=0}^{n-1}\left\{ \left(-1\right)^{k+1}C\left(x+n,y+n-k\right)-\left(-1\right)^{k}C\left(x+n,y+n-1-k\right)\right\} \\ & =\left(-1\right)^{n}C\left(x+n,y\right)-\sum_{k=0}^{n-1}\left(-1\right)^{k+1}\left\{ C\left(x+n,y+n-k\right)+C\left(x+n,y+n-1-k\right)\right\} \\ & =\left(-1\right)^{n}C\left(x+n,y\right)-\sum_{k=0}^{n-1}\left(-1\right)^{k+1}\left\{ C\left(x+n+1,y+n-k\right)\right\} \\ & =\left(-1\right)^{n}C\left(x+n,y\right)+\sum_{k=0}^{n-1}\left(-1\right)^{k}C\left(x+n+1,y+n-k\right)\\ & =\left(-1\right)^{n}C\left(x+n,y\right)+\sum_{k=0}^{n-1}\left(-1\right)^{n-1-k}C\left(x+n+1,y+1+k\right)\\ & =\left(-1\right)^{n}\left\{ C\left(x+n,y\right)-\sum_{k=0}^{n-1}\left(-1\right)^{k}C\left(x+n+1,y+1+k\right)\right\} \end{align*}ページ情報
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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2乗和
\[
\sum_{j=0}^{m}C^{2}(m,j)=C(2m,m)
\]
負の整数の2項係数
\[
C\left(-m,-n\right)=\left(-1\right)^{m-n}C\left(n-1,m-1\right)
\]
2項係数が0になるとき
\[
\forall m,n\in\mathbb{Z},\left(0\leq m<n\right)\lor\left(n<0\leq m\right)\lor\left(m<n<0\right)\Leftrightarrow C\left(m,n\right)=0
\]