Normal sudoku rules apply. Each line contains a square number reading from left to right (or in the case of the line in r3c1, from top to bottom). eg If r8c7 was a 2 then r7c8 must be a 5 (to give the square number 25). (As an aide memoire, all 2-digit squares and 3-digit squares (without zeroes or a repeated digit) are: 16, 25, 36, 49, 64, 81, 169, 196, 256, 289, 324, 361, 529, 576, 625, 729, 784, 841, 961).