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Number theory Difficulty 4.5 AIME Prove it Romania

Prove that all positive integers, except the powers of 22, can be written as the sum of (at least two) consecutive positive integers.

Solution

All symbols in the sequel are denoting integer numbers. Let n=2abn = 2^a b, a0a \ge 0, b1b \ge 1, bb odd. We want to have n=(m+1)+(m+2)++(m+k)n = (m+1) + (m+2) + \dots + (m+k), with m0m \ge 0 and k2k \ge 2, hence k(2m+k+1)=2a+1bk(2m+k+1) = 2^{a+1}b.

If b=1b=1, it follows k=2αk=2^\alpha, with 1αa+11 \le \alpha \le a+1, but then 2m+k+1>12m+k+1 > 1 is odd, so there are no solutions. Now, for b>1b > 1, we can exhibit the required writing.

If b2a+1+1b \ge 2^{a+1} + 1, then take m=12(b2a+11)m = \frac{1}{2}(b - 2^{a+1} - 1) and k=2a+1k = 2^{a+1}.

If b2a+11b \le 2^{a+1} - 1, then take m=12(2a+11b)m = \frac{1}{2}(2^{a+1} - 1 - b) and k=bk = b.

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