Mostly code and coment on software, with the occasional social post.

Things I want to know about Sudoku!

Is there an initial configuration of the board which looks sensible, but has a logical inconsistency so there aren’t actually any solutions.

In the puzzles posed by newspapers (at least the easy ones I have done), there is normally always a fully constrained position for every next move. Is this always the case?

If there isn’t a fully constrained position for every next move, are all subtrees except one blind alleys, or not?

How does one generate a puzzle where one knows that there are always enough constraints to make a reasonably unambiguous decision?

In the case of underconstrained configurations how large is the state space?

In the case of no initial constraints (no numbers at all), how large is the state space of all possible solutions?