Sudoku 8 - Evil

SUDOKU #8

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 1 in BR (because of CK and AO). There is a 4 in EO (because of BM, GN, and FP) and a 7 in FO (because of HM and CN). There is a 4 in DK (because of CJ, EO, and FP) and one in IL (because of CJ, DK, and GN).

The 3 in HK and BP means that there has to be a 3 in either IQ or IR, so the 3 in box GM-IO has to be in GO. There is a 3 in AN (because of FM, GO, and BP). There is another 3 in CL (because of AN and BP).

There is an 8 in GQ and one in DO, so the 8 in column M has to be in either AM or CM. There is a 2 in BQ and another in HO, so there has to be a 2 in either AM or CM. That means that the 5 and 6 in box AM-CO have to be in BN and CO. The 2 in BQ means that the 2 in box AJ-CL has to be in for A, so the 2 has to be in CM and the 8 has to be in AM.

The missing numbers from column M are 5, 6, and 9. But there are a 6 and 9 in row E, so a 5 is in EM.

The numbers missing from box AM-CO are the 5 and 6. The numbers missing from row C are 5, 6, 8, and 9. But there is an 8 and a 9 in column Q, so the possible numbers in CQ are 5 and 6 -- the same as in CO. So the possible numbers in CP and CR are the 8 and 9. The numbers missing from column O are a 5 and 6, and the numbers missing from column M are a 6 and 9.

This puzzle is hard enough that I am trying to see patterns. Are there a couple squares in a row, column or box that have to be one of two numbers, so we can get more information about other squares?

The numbers missing from row B are 5, 6, 7, 8. The numbers missing from row C are 5, 6, 8, 9. The numbers in row F are 1, 5, 6, 8, 9. There are a 1 and 5 in column R and a 9 in box DP-FR, so the possible numbers in FR are a 6 or 8.

5678

5689

15689

12679

14568

The numbers missing from box AP-CR (aside from the 8 and 9) are 4, 5, 6, and 7, but there is a 5 and a 7 in column R. So the possible numbers in AR would be 4 and 6. The numbers missing from row H are 1, 4, 5, 6, and 8, but there are a 1 and 5 in column R and an 8 in box FP-IR. So the possible numbers in HR would be a 4 and a 6. So the 4 and 6 have to be in AR or HR. That means that there cannot be a 6 in FR, which means that the 8 has to be in FR. That means there has to be a 9 in CR and an 8 in CP.

5678

5689

15689

12679

14568

There is a 2 and a 3 in ER and IR. There is a 1, 2, or 3 in EN, EP, and ER, but there is a 3 in AN and in BP. So the 3 has to be in ER, and that means the 2 is in IR. There has to be a 3 in DJ (because of ER and FM). There has to be a 7 and 8 in EJ and EL (because of DO, FR, FO, and DR).

5678

1569