公式

Pkn=n!(n−k)!=nPk−1n−1=Pkn−1+kPk−1n−1

Ckn=n!k!(n−k)!=Ck−1n−1+Ckn−1

D(n)=(n−1)[D(n−2)+D(n−1)]

ap−1≡1(modp)

Fn=15√[(1+5√2)n−(1−5√2)n]

∑ni=1i2=n(n+1)(2n+1)6

∑ni=1(2i−1)2=n(4n2−1)3

∑ni=1i3=n2(n+1)24

∑ni=1(2i−1)3=n2(2n2−1)

∑ni=1n(n+1)=n(n+1)(n+2)3

week=(day+2month+3(month+1)5+year+year4−year100+year400)%7+1