f(n,k)=∑i=0n(−1)i(ni)min(i,k)f(n,k)=\sum_{i=0}^n(-1)^i\binom{n}{i}\min(i,k)f(n,k)=∑i=0n(−1)i(in)min(i,k) 由打表可得: f(n,k)=(−1)k(n−2k−1)f(n,k)=(-1)^k\binom{n-2}{k-1}f(n,k)=(−1)k(k−1n−2) 求证明。