Problem:
Let be the sum of elements of a nonempty finite set , where is a set of numbers. Calculate the sum of all numbers where ranges over all nonempty subsets of the set .
Solution
Solution:
The answer is
We note that each belongs to subsets of . We reason as follows: we can assign 0 or 1 to according to whether it is not or in a subset of . As there are 2 choices for a fixed , belongs to half of the total number of subsets, which is . Hence the sum is
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