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k-special numbers

Source: Lusophon Mathematical Olympiad 2023 Problem 3

May 27, 2023
number theory

Problem Statement

An integer nn is called kk-special, with kk a positive integer, if it's the sum of the squares of kk consecutive integers. For example, 1313 is 22-special, since 13=22+3213=2^2+3^2, and 22 is 33-special, since 2=(1)2+02+122=(-1)^2+0^2+1^2.
a) Prove that there's no perfect square that is 44-special.
b) Find a perfect square that is I2I^2-special, for some odd positive integer II with I3I\ge 3.