XY-Chains
The Y-Wing Chains are infact part of a more encompassing strategy called XY-Chains. The commonality is the same pincer-like attack on candidates that both ends can see and that the chain is made of bi-value cells. With Y-Chains the hinge was expanded to a chain of identical bi-value cells but in an XY-Chain these can be different - as long as there is one candidate to make all the links. The "X" and the "Y" in the name represent these two values in each chain link.The example here is a very simple XY-Chain of length 4 which removed all 5's highlighted in yellow. The chain ends are 5 A7 and C2 - so all cells that can see both of these are under fire. It's possible to start at either end but lets follow the example from A7. We can reason as follows
- If A7 is 5 then A3/C7/C9 cannot be.
- if A7 is NOT 5 then it's 9, so A5 must be 2, which forces A1 to be 6. If A1 is 6 then C2 is 5.
https://www.sudokuwiki.org/sudoku.htm?bd=080103070090506000001408020578241639143659782926837451037905200000304097419782060
C2的candidates是56
A1的candidate是26
A5的candidates是29
A7的candidates是59
XY-Chain
length=4, chain ends: A7 and C2
This proves 5 is the solution at one end of the chain or the other
-5[A7]+9[A7]-9[A5]+2[A5]-2[A1]+6[A1]-6[C2]+5[C2]
5 taken off A3
5 taken off C7
5 taken off C9
标签:sudoku,chain,A1,XY,A7,C2,Chains From: https://www.cnblogs.com/chucklu/p/18536830