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Easy count (Posted on 2014-07-29) Difficulty: 3 of 5
Let S be a set of some positive integers.
We'll call S autonomous if the number of elements in S is itself an element of S. e.g. the set {2,3,5} is autonomous, as is the set {2,7}, but the sets {1, 4} or {2,4,5} are not.

Determine a general formula for the number
of autonomous subsets of {1, 2, 3, ... , n}.

See The Solution Submitted by Ady TZIDON    
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re: Solution | Comment 4 of 5 |
(In reply to Solution by tomarken)

Indeed I misread and found the number of non-autonomous subsets.
  Posted by Charlie on 2014-07-30 09:57:26

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