Update

sudoku.md

@@ -53,7 +53,8 @@ Humans almost exclusively use logical reasoning techniques to solve sudoku, whic
 
 - **scanning**: We take a look at some frequently appearing number in the grid and see which columns and rows they intersect which implies they cannot be placed in those columns and rows, possibly revealing the only possible location to place such number.
 - **single remaining candidate**: When there is only one number left to fill in any column, row or subgrid, it is always clear which one it is and can be safely placed.
-- **candidate sets**: An advanced technique in which we create sets of possible candidate numbers for each square on the grid e.g. by writing tiny numbers in the top corners of the squares. We then apply various reasoning to reduce those sets, i.e. remove candidate numbers, until a single candidate remains for a certain square in which case we can fill in that number with certainty. This will further help us reason about candidates in other squares.
+- **candidate sets**: A more advanced technique in which we create sets of possible candidate numbers for each square on the grid e.g. by writing tiny numbers in the top corners of the squares. We then apply various reasoning to reduce those sets, i.e. remove candidate numbers, until a single candidate remains for a certain square in which case we can fill in that number with certainty. This will further help us reason about candidates in other squares.
+- **inference chains and cycles**: Very advanced techniques, described e.g. at http://zitowolf.net/sudoku/strategy.html. { TBH no idea what this is. ~drummyfish }
 
 For computers the traditional 9x9 sudoku is nowadays pretty easy to solve, however solving an NxN sudoku is an [NP complete](np_complete.md) problem, i.e. there most likely doesn't exist a "fast" algorithm for solving a generalized NxN sudoku, even though the common 9x9 variant can still be solved pretty quickly with today's computers by using some kind of "smart" [brute force](brute_force.md), for example [backtracking](backtracking.md) (or another state tree search) which [recursively](recursion.md) tries all possibilities and at any violation of the rules gets one step back to change the previous number. Besides this a computer can of course use all the reasoning techniques that humans use such as creating sets of possible values for each square and reducing those sets until only one possibility stays. The approach of reasoning and brute forcing may also be combined: first apply the former and when stuck fall back to the latter.