I haven't done math problems for a long time. U is actually a set of 1 9 positive integers from1to 19. The condition of set A actually means 1. 1

If a has 19 possible elements, then the sum of its nonempty sets is 2 19 power-1.

Well, I think this is the examination center. Let me explain something to you. For example, if A contains two elements, A and B, then the number of non-empty subsets of A should be the second power of 2-1=3, and the minus 1 of {a}{b}{a, b} is an empty set. The three elements a, b and c are the cubic of 2-1 = 7 {a} {b} {c} {a, b} {a, c} {b, c} {a, b, c} {a}. By analogy, suppose that A contains n elements and A has 2 n elements.