Tobymorton wrote:Could you possible recommend a good article that clearly explains these advanced chain correlations

Some general chaining concepts (including XY-Chains):

http://hodoku.sourceforge.net/en/tech_chains.phpXY-Chains specifically:

http://www.sudokuwiki.org/XY_ChainsAlternating Inference Chains (AIC):

http://forum.enjoysudoku.com/an-aic-primer-t33934.htmlpjb wrote:The 4 member XY chain:

(2=3)r5c8 - (3=9)r5c2 - (9=4)r7c2 - (4=2)r7c9 => -2 r5c9

Phil's XY-Chain is one kind of an AIC. The basic idea of any AIC is that it establishes a derived OR-relationship (strong link) between the two end candidates (or nodes to be more general), which means that at least one of them has to be true. Thus any other candidate that (if true) would force both of those ends to be false, i.e. has a weak link with both of them, can be eliminated. An AIC has alternating strong and weak links, with strong links at both ends. In the Eureka chain notation which is mostly used here as in Phil's example, the strong links are marked with "=" and weak links with "-". All strong links in a pure XY-Chain are bivalue cells, i.e. cells with just two candidates left.

Phil's chain proves that either the cell in [row 5, column 8] must be 2 or the cell in [row 7, column 9] must be 2 (or both); thus there can be no 2 in [row 5, column 9]. You can check the result by trying what would happen if you placed 2r5c9: it would cause a contradiction.