http://oeis.org/A001499中的(PARI) a(n)=if(n<2, n==0, (n^2-n)*(a(n-1)+(n-1)/2*a(n-2)))怎么理解?
测试 N=20 M=1000000007 s=[i for i in range(1,N+1)] s[1]=3 for i in range(2,N): s[i]=s[i-1]+s[i-2] if(s[i]>M): s[i]=s[i] % M for j in range(1,N-1): for i in range(j+1,N): s[i]+=s[i-1] if(s[i]>M): s[i]=s[i] % M print(s[N-1] % 1000000007)