Does every validly posed Sudoku (doesn't break any Sudoku rules so only no duplicate 1 to 9 in rows, columns or square) have at least one solution, and if not, what is the minimum number of givens for it to be unsolvable?
According to Wikipedia:
The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a single solution.
Many others sources including Sudopedia do not state that a Sudoku-like grid with given 1 to 9 numbers has to have one solution to be a Sudoku.
Does have given numbers mean it is solvable, and assuming no, what is the least numbers in a validly posed grid needed to be for it be unsolvable?