Euler may not have cracked this problem completely, but it led to a lot of important work, including on what we today know as sudoku.

Get on a commuter train these days and you can virtually see people's brains crunching away at filling the numbers from 1 to 9 into a square grid. As the Sudoku craze shows no sign of slowing, Hardeep Aiden investigates its relatives and predecessors.