A magic square is a grid of numbers in which every row, column and diagonal adds up to a square number

Explanation

In a magic square the sums are all equal, but that shared value is called the magic constant, and it does not have to be a square number. For a square built from the integers 1 to n², the constant is n(n² + 1) / 2. For the classic 3×3 square using 1 through 9 that gives 3 × 10 / 2 = 15. That famous square, the Lo Shu square, is unique up to rotations and reflections, and legend says it was first seen inscribed on the shell of a turtle rising from the Luo River in China.