Sudoku 10 - Evil

SUDOKU #10

I'm going to repeat the info at the top of Sudoku #1, just in case you haven't seen that one yet.

I've been doing Sudoku for a while. There are a lot of people like me who have figured out some tricks or techniques that help them complete the puzzles. This page is not really for them. I met a woman recently who was just starting to try to do Sudoku. This page is for people like her. I'm not saying that I know every single trick, but I know enough to complete most puzzles.

The basic rules of Sudoku are that there is only one of each number in each row, column, and 9 square grid. There isn't any mathematics or arithmetic. If you know how to count from 1 to 9, then you can do these puzzles. All of the techniques are based on this rule.

Here is a sample Sudoku puzzle.

This is a Evil difficulty puzzle (harder than Hard), and it will allow me to show some examples of a few of the tricks that I've uncovered. I've added the letters below to the rows and columns in order to talk about each of the 81 squares. As a convention, I will talk about squares and boxes. There are 81 squares, but there are 9 boxes of 9 squares.

There's a 9 in AR (because of GP, EQ, and CL), another 9 in BM (because of AR, CL, and IN). There's a 5 in BJ (because of HL and FK) and another 5 in CP (because of BJ, AN, and ER).

There's a 3 in AP (because of DQ and IR) and another one in CK (because of AP and BO). The 3s in CK and IR mean there has to be a 3 in either GJ or GL, so there has to be a 3 in HN (because of IR, CK, and BO). There has to be a 9 in HK (because of GP and IN).

There is a 7 in HO. You can see that in two ways. First, the 7 in GR means the 7 in box GJ-IL has to be in row I. Second, the numbers missing in row H are 2, 4, 7, and 8. All but one of the squares of row H are in box GP-IR, but there is a 7 in GR, so the 7 has to be in HO.

The 4 in BP and the 6 in BR mean that there has to be a 4 and 6 in AK and AL, which means that a 7 and an 8 have to be in BK and BL. The 3 in DQ and 7 in DP mean that there has to be a 3 and 7 in EL and FJ. There is a 6, 8, and 9 in squares DJ-DL, so the 9 has to be in DJ (because of CL and HK).

There is a 9 in FO (because of BM, DJ, EQ, and IN). There's a 7 in AM (because of BK-BL, CQ, and HO) and another one in FN (because of AM, HO, and DP).

The 7 in FN means there is a 3 in FJ and a 7 in EL. The numbers missing from column J are a 7 and 8. The 7 in GR means there is a 7 in IJ and an 8 in GJ. The 7 in EL means there is a 7 in BK, an 8 in BL and DK, a 6 in DL and AK, and a 4 in AL.

There's a 4 in IK. There's a 3 in EM (because of DQ, FJ, and BO). There's a 3 in GL (because of IR) and a 1 in IL, a 1 in GQ (because of IL and HM), a 1 in CR (because of GQ), a 1 in FP (because of CR, HQ, and EK), a 1 in BN (because of AJ and CR), and a 1 in DO (because of EK, HM, and BN). That's all the 1s.

There's a 2 in AO, because the numbers missing from row C are the 4, 6, and 8. So there's an 8 in AQ, a 2 in BQ. The numbers missing from row E are a 2 and 6, but the 2 in AO means the 2 is in EP and the 6 is in EO.

&nb