Consecutive Square-Infected Positive Integers

There are k k consecutive positive integers, none of which is square-free.

What is the sum of all possible values of k 2017 k\leq2017 ?


Details and Assumptions:

  • A positive integer n n is square-free if there is no positive integer k > 1 k>1 such that k 2 n k^2 \Big|\, n .

  • Before solving this, you may want to solve this and this .


The answer is 2035153.

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1 solution

Actually,

For every positive integer k k , there are k k consecutive positive integers, none of which is square-free.

Let's demonstrate a constructive proof.

Proof: Solve the following system of k k linear congruences using the Chinese Remainder Theorem : x i ( m o d p i 2 ) , for 1 i k , x \equiv -i (\mod p_i^2) \text{, for } 1\leq i \leq k, where each p i p_i is distinct prime. Assume the solution is: x X ( m o d M ) x\equiv X (\mod M) , where M = 1 k p i 2 M=\prod_1^k p_i^2 , and X X is a non-negative integer.

Then, ( X + 1 ) , ( X + 2 ) , (X+1),(X+2),\cdots{ }\cdots{ } \cdots{ } and ( X + k ) (X+k) are k k consecutive positive integers, none of which is square-free, because for every i i (with 1 i k 1\leq i \leq k ), p i 2 ( X + i ) p_i^2 \mid (X+i) . \blacksquare

So the sum of all possible values of k = 1 + 2 + 3 + + 2017 = 2017 × 2018 2 = 2035153 k = 1+2+3+\cdots{ }\cdots{ } \cdots{ }+2017=\frac{2017\times 2018}{2}=\boxed{2035153} .

Nice problem and solution! I wasn't aware of this result before. It took until my third attempt before thinking of using the CRT.

Brian Charlesworth - 4 years, 5 months ago

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