The number 1 has this property, since the only positive integer not exceeding 1 is 1 itself, and therefore the
sum is 1. This is a constructive proof. (查看原文)
Many people fnd it confusing that "p only if q" expresses the same thing as "if p then q." To remember this, note that "p only if q" says that p cannot be true when q is not true. That is, the statement is false if p is true, but q is false. When p is false, q may be either true or false, because the statement says nothing about the truth value of q. A common error is for people to think that "q only if p" is a way of expressing p → q. However, these statements have different truth values when p and q have different truth values. (查看原文)
