The sum of consecutive positive integers is always odd .
Find the number of positive integers such that─
There is at least one sequence of consecutive positive integers with even sum.
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First, consider the case where n is odd. A sequence of n consecutive integers can have either 2 n − 1 or 2 n + 1 odd integers. If n ≡ 1 m o d 4 , then 2 n − 1 is even, and if n ≡ − 1 m o d 4 , then 2 n + 1 is even. If the number of odd integers in the sum is even, then the sum itself is even, so all odd n are to be included.
Now consider even n . In the sequence of n consecutive integers, we will always have exactly 2 n odd integers. If n ≡ 0 m o d 4 , then 2 n is even and the sum will always be even. However, if n ≡ 2 m o d 4 , then 2 n is odd, and the sum will always be odd, as it contains an odd number of odd integers. Thus, the only integers NOT to be included in our count are those where n ≡ 2 m o d 4 , of which there are 5 0 4 less than 2 0 1 7 (this can be calculated as 4 2 0 1 4 − 2 + 1 ), leaving 2 0 1 7 − 5 0 4 = 1 5 1 3 integers such that there is at least one sequence of n consecutive positive integers with even sum.