Sudoku 9 - Evil

SUDOKU #9

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 is a 9 in AL (because of CM and BQ). There is a 5 in HL (because of GO and IQ). There is a 6 in FO (because of DJ, EQ, and HM) and another 6 in AN (because of CR and HM).

There has to be a 4 in either AM or BM (because of CJ and FN). There has to be a 4 or 9 in HO and IO (because of the 4 in AM-BM and FN, and because of the 9s in CM and DN). There's an 8 and 9 in EJ and FK (because of AL, DN, DR, and IL).

There's a 2 in BJ (because of HK). So there is a 5 or 8 in BK and CK (because of HL and IL). That means that there is an 8 in EJ and a 9 in FK. And there's a 6 and 7 in BL and CL, which means there is a 6 in BL (because of CR) and a 7 in CL. The missing numbers in column J are the 7 and 9.

There is a 9 in EP (because of FK, DN, and GR). There's an 8 in FM (because of DR and EJ) and another one in CO (because of FM). That means there is an 8 in BK and a 5 in CK. There's a 2 and a 3 in DO and EO. There's a 7 in DM (because of AO and EK).

There's a 1 and a 5 in EM and EN. Therefore, there's a 5 in DP (because of EM/EN, FJ, and IQ), and there's a 1 in DL (because of EM/EN, FR, and AK).

The numbers missing from box GM-IO are 1, 2, 7, 8. There is a 1 in GJ and a 7 and 8 in column M, so there has to be a 2 in GM. That means that there has to be a 2 in CN (because of GM and DO/EO) and another 2 in AQ (because of IR, BJ, and CN).

There's a 3 in BN (because of AJ and IM). That means that there is a 4 and a 5 in AM and BM -- and AR, which means EM has to be a 1 and EN has to be a 5.

The numbers missing from row D are the 2, 3, and 4, but there is a 2 in HK and AQ, so the 2 has to be in DO -- which means the 3 is in EO. There's another 2 in FP (because of IR and AQ). So there is a 2 in EL and a 4 in ER. That means there is a 4 in DK, a 3 in DQ, a 4 in ER, a 7 in FQ, a 3 in FL, and a 4 in GL.