Puzzle #17 help, Hint #4

[wzee]

I don't understand how you can continue to place 3's and 7's at Hint #4.. I have them on the left and right boxes respectively, and in the center box, but then I was unable to continue. What am I missing?

[dave.kohler]

I suspect you are missing a number of opportunities to apply the knight and king rules in all cases. for example if you have just 2 adjacent cells in a 3x3 box that are still pencil marked, the can reach by knight move into an adjacent 3x3 box and eliminated those same pencil marks there. For example if the only cells pencil marked for 3 in box 1 are in R3C1/c2 then they not only can eliminate pencil marks for 3 in the remainder of R3, but also they eliminate pencil marks for 3 in R5C1/C2.

If only two cells are pencil marked for a value in a given row or column, and both can see a nearby cell in a nearby row or column via the knight or king constraint, then those nearby cells can have their pencil marks remove.

Using just these considerations you can get all the 3s and 7s except in box 2 and 8,

BTW, the app does NOT automatically remove ALL appropriate pencil marks when a value is placed in a cell. It does remove all EXCEPT those adjacent cells still can retain pencil marks for consecutive values. For example when I place 3 in a cell, I must also manually remove pencil marks for 2 and 4 from the cells immediately beside,above,and below where the 3 was placed.

This should help. BTW it still takes me about 30 minutes to complete #17, so don't feel bad.