**Warning:**Today’s puzzle is EXTREMELY difficult! Basically, not only do you have to fight against 5 interconnected Sudokus, but you also need to be carefull about the diagonals:

**ALL**diagonals in the puzzle must contain one occurrence of each 1-9 digit. The diagonals are marked in the puzzle so you wouldn’t forget about them.

(click to download or right-click to save the image!) | |

**To see the solution to this puzzle**click here

## 5 Comments

I’m completely blocked!!! I can’t go further!!!There are special techniques envolved to solve this puzzle? Best regards,

Fernando Castro

Fernando, there are no special techniques required to solve this one. However, it does require some of the more difficult classic Sudoku techniques – namely naked and hidden pairs.

I can also tell you this: you can solve 4 out of 5 sudokus (all except bottom right) by using only naked/hidden singles and row/column/nonet interactions.

For the bottom right sub-puzzle, you MUST use naked/hidden pairs.

Lucky I didn’t post a Samurai that requires Swordfish! ðŸ™‚

I know that this is a site not a school but I need urgently an explanation about naked/hidden singles/pairs! Probably I use them already but … I don’t ‘see’! Can you help me?

Fernando Castro

P.S. I have already bought your book in Lisbon but I didn’t start yet! It is a Christmas gift!

Fernando, naked singles are your simplest technique that you already use: you find one cell from which you eliminate 8 numbers for one reason or another end the remaining 9th number must be the solution for that cell. Hidden singles are when one number (lets say number 1) is eliminated from 8 cells in one column (for example), so that number must go into the 9th cell.

Naked pairs are similar: you find 2 cells in a row that have only 2 numbers as candidates (other 7 have been eliminated). Those 2 numbers MUST BE the same for both cells. Well, you can’t solve those two cells, but you can eliminate those two numbers from all other cells in that row.

Hidden pairs are when 2 numbers are eliminated from 7 cells in a row, so they must go into the remaining two cells in that row. Once you figured that, then those 2 numbers become naked pairs – you can eliminate all other numbers from those 2 cells.

You can read more about that here:

http://www.simes.clara.co.uk/programs/sudokutechniques.htm

Wow! That was tough!

It took me a day to complete, but was rewarding once I’d finished. I found I had to do a lot of forward projections to rule out certain values; once the centre Sudoku was complete, it was pretty straightforward to finish.

A while ago, I developed a spreadsheet that can help you with the basic logic. Find it here:

http://www.osirra.com/resource/1/36

I didn’t resort to using it on this one.

Thanks ðŸ™‚

Dan.