@MTabarrok Proof by induction is easy to reword as proof by contradiction.
"P(0) is true, P(x) implies P(x+1), therefore P(any +ve int) is true"
Becomes
"suppose P(x) is not always true. Let z be smallest x where P(z) is false. But then P(z-1) is true, so P(z) is true, contradiction!"
@归纳法证明很容易改写为矛盾证明。
P（0）为真，P（x）意味着P（x+1），因此P（any+ve int）为真
变成
“假设P（x）不总是真的。设z为最小的x，其中P（z）为假。但是P（z-1）为真，所以P（z）为真，矛盾！”