2項係数の半分までの総和

2項係数の半分までの総和

(1)偶数の場合で半分以下

k=0n1C(2n,k)=22n112C(2n,n)

(2)偶数の場合で半分以上

k=0nC(2n,k)=22n1+C(2n,n)

(3)奇数の場合で丁度半分

k=0n1C(2n1,k)=22n2

(1)

k=0n1C(2n,k)=12(k=0n1C(2n,k)+k=0n1C(2n,2nk))=12(k=0n1C(2n,k)+k=0n1C(2n,2n(n1k)))=12(k=0n1C(2n,k)+k=0n1C(2n,n+1+k))=12(k=0n1C(2n,k)+k=n+12nC(2n,k))=12(k=02nC(2n,k)C(2n,n))=22n112C(2n,n)

(2)

k=0nC(2n,k)=k=0n1C(2n,k)+C(2n,n)=22n112C(2n,n)+C(2n,n)=22n1+C(2n,n)

(3)

k=0n1C(2n1,k)=12(k=0n1C(2n1,k)+k=0n1C(2n1,2n1k))=12(k=0n1C(2n1,k)+k=0n1C(2n1,2n1(n1k)))=12(k=0n1C(2n1,k)+k=0n1C(2n1,n+k))=12(k=0n1C(2n1,k)+k=n2n1C(2n1,k))=12(k=02n1C(2n1,k))=22n2
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