Y by
Prove that for every natural number
, at least one of the integers
is not a perfect square.

![\[ 2k-1, \quad 5k-1 \quad \text{and} \quad 13k-1\]](http://latex.artofproblemsolving.com/1/0/e/10eda7122d397a502fa4b635ac7062a2022aee8c.png)
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