12
votes
1answer
941 views

Minimum number of different clues in a Sudoku

I wonder if there are proper $9\times9$ Sudokus having $7$ or less different clues. I know that $17$ is the minimum number of clues. In most Sudokus there are $1$ to $4$ clues of every number. ...
5
votes
3answers
305 views

How complicated is the set of tautologies?

Consider the set $\mathcal T$ of all tautologies in the propositional calculus in which the only operators allowed are $\to$ and $\neg$, and involving only the two variables $x$ and $y$. How ...