Would you count it manually? Part 2

How many subsets of { 1 , 2 , 3 , , 39 , 40 } \left\{ {1,2,3, \ldots ,39,40} \right\} contain at least two consecutive integers?

Image Credit: Flickr Mervyn Chua .


The answer is 1099243713480.

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1 solution

Maggie Miller
Aug 5, 2015

Count the number of subsets with no consecutive integers:

For k 40 k\le 40 , choosing k k elements, none of which are consecutive, is equivalent to placing k k bars in distinct places between 40 k 40-k balls. Therefore, there are ( 41 k k ) {41-k \choose k} possibilities.

Therefore, the answer is 2 40 k = 0 40 ( 41 k k ) = 2 40 k = 0 20 ( 41 k k ) = 2 40 F ( 42 ) \displaystyle2^{40}-\sum_{k=0}^{40}{41-k \choose k}=2^{40}-\sum_{k=0}^{20}{41-k \choose k}=2^{40}-F(42) , where F ( 42 ) F(42) is the 4 2 nd 42^{\text{nd}} Fibonacci number.

With a calculator, we evaluate and get 1099243713480 \boxed{1099243713480} .

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