The New Sudoku Players' Forum

[turbo012]

i got this puzzle from the internet and i thought it was pretty hard until i looked at some sites.

i see the sites using special methods i guess as a way to solve it faster.

but i don't understand most of them so here's the puzzle and i hope i can understand how to solve from top to bottom logically as possible.

Code: Select all

   |52 |63
 5 |   | 7
9  |  8|  2
---+---+---
 17|  4|   
 9 |   | 6
   |8  |31
---+---+---
1  |6  |  5
 4 |   | 9
 86| 95|   


in the beginning i only got to this point before i had to look up the internet.

Code: Select all

 7 |52 |63
 5 |   | 7
9  |  8|  2
---+---+---
 17|  4|   
 9 |   | 6
   |8  |31
---+---+---
1  |6  |  5
 4 |   | 96
 86| 95|  3


note: i usually solve these on paper, but i'm wondering when you guys solve these if you always start off writing 1-9 on the blanks cells.

[ravel]

With singles you come here:

Code: Select all

--------------------------------------------------------
48     7   148    | 5     2      19    | 6     3    1489       
23468  5   12348  | 1349  1346   1369  | 489   7    1489       
9      36  134    | 1347  13467  8     | 45    45   2   
--------------------------------------------------------
23568  1   7      | 239   356    4     | 2589  258  89         
23458  9   23458  | 1237  1357   1237  | 2458  6    478       
2456   26  245    | 8     567    2679  | 3     1    479   
--------------------------------------------------------
1      23  9      | 6     3478   237   | 2478  248  5         
2357   4   235    | 1237  1378   1237  | 1278  9    6         
27     8   6      | 1247  9      5     | 1247  24   3         
--------------------------------------------------------


From the pair 45 in r3c78 you can eliminate 4 from r1c9,r2c7,r2c9,r3c3,r3c4,r3c5.
Then box elimination: In row 1 only r1c13 have candidate 4 left (which gives you only these 2 possibilities for a 4 in box 1) - eliminate 4 from r2c1,r2c3.
The rest is singles again.

See here for the techniques needed:
http://www.simes.clara.co.uk/programs/sudokutechniques.htm
http://www.angusj.com/sudoku/hints.php

[turbo012]

With singles you come here:

Code: Select all

--------------------------------------------------------
48     7   148    | 5     2      19    | 6     3    1489       
23468  5   12348  | 1349  1346   1369  | 489   7    1489       
9      36  134    | 1347  13467  8     | 45    45   2   
--------------------------------------------------------
23568  1   7      | 239   356    4     | 2589  258  89         
23458  9   23458  | 1237  1357   1237  | 2458  6    478       
2456   26  245    | 8     567    2679  | 3     1    479   
--------------------------------------------------------
1      23  9      | 6     3478   237   | 2478  248  5         
2357   4   235    | 1237  1378   1237  | 1278  9    6         
27     8   6      | 1247  9      5     | 1247  24   3         
--------------------------------------------------------


From the pair 45 in r3c78 you can eliminate 4 from r1c9,r2c7,r2c9,r3c3,r3c4,r3c5.
Then box elimination: In row 1 only r1c13 have candidate 4 left (which gives you only these 2 possibilities for a 4 in box 1) - eliminate 4 from r2c1,r2c3.
The rest is singles again.

See here for the techniques needed:
http://www.simes.clara.co.uk/programs/sudokutechniques.htm
http://www.angusj.com/sudoku/hints.php

how did you deduce of eliminating the number 4 from r1c9,r2c7,r2c9,r3c3,r3c4,r3c5?

what did you mean by r1c13?

and also how did you deduce that 4 can't be in r2c1,r2c3?

i'm trying to learn the logic behind it but only thing i can think of is guess and check couple steps ahead.

[ravel]

how did you deduce of eliminating the number 4 from r1c9,r2c7,r2c9,r3c3,r3c4,r3c5?

If you follow the 2nd link above to Angus' site, you can read it under "Naked pairs" ("naked subset" on Simes page): the 45 pair is a pair both in box 3 and row 3.

what did you mean by r1c13?

One of the cells r1c1 or r1c3.

and also how did you deduce that 4 can't be in r2c1,r2c3?

Read "Locked candidates 2" ("Block/Block Interaction").

[turbo012]

thanks alot i guess i should try to memorize the steps and practice them.

on a last note... do you start off a puzzle by writing 1-9 on the blank cells?

[ravel]

No, on paper i fill in the candidates as late as possible. Also some pairs and even x-wings can be spotted without "candidate counting". When i do it (i would have needed it to spot the 45-pair in your puzzle), i start with the cells where obviously only few candidates remain (already much numbers in the same row/column/box) and try to get further with them before filling the candidates in all the other cells.