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What constitutes a valid sudoku puzzle?

A sudoku puzzle should:

• Have a single unique solution
• Be solvable using logic alone
• Be on a square grid
• Have the numbers 1 to 9, once per row, column, and 3 x 3 square (for a 9x9 puzzle)

More controversially, many people would also say that a 'proper' sudoku puzzle should also have symmetry of some sort. The most common is the two way type of symmetry down one of the diagonals, though it could also be reflectional, four way or even eight way symmetry.

Any other puzzle is either not a sudoku puzzle or a valid sudoku puzzle.

For instance, many grids have more than one possible solution. The total number of valid sudoku grids may appear at first to be quite small, but in fact there are an incredibly high number of possible solutions - we challenge you to do the math and work out just how many valid sudoku grids there are!