How many subsets of
contain at least two consecutive integers?
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Count the number of subsets with no consecutive integers:
For k ≤ 4 0 , choosing k elements, none of which are consecutive, is equivalent to placing k bars in distinct places between 4 0 − k balls. Therefore, there are ( k 4 1 − k ) possibilities.
Therefore, the answer is 2 4 0 − k = 0 ∑ 4 0 ( k 4 1 − k ) = 2 4 0 − k = 0 ∑ 2 0 ( k 4 1 − k ) = 2 4 0 − F ( 4 2 ) , where F ( 4 2 ) is the 4 2 nd Fibonacci number.
With a calculator, we evaluate and get 1 0 9 9 2 4 3 7 1 3 4 8 0 .