Unsolvable Su Dokus

[dr john]

Following on from a question below, is it always possible to solve a Su Doku without at some point having to guess? I do The Times, and occasionally (not always at the 'fiendish' level) reach a point where no amount of logic will get me any further.
I then try to find a 50/50 choice to pick at random, and it's often 5 or 6 moves further on before I know whether I've made the right choice. This seems beyond the power of mere human calculation - or is there some pons asinorum I've yet to cross? (I noticed the winner of last year's world championship admitted she had to guess at one point.)

[kayakamina]

As someone who contributed to the post below I find that I am able to solve most puzzles through logic but resort to guessing with the 'fiendish' type.
What i do not subscribe to is the myth that there is only one solution for each puzzle. The Telegraph's MM continues this and even though I have emailed differing solutions to one of his sudoku's he persists.

[Figi]

I do the Times ones as well and I have to admit that I sometimes have to resort to the 50/50 technique. However, I'm willing to be persuaded that it is possible to do them without resorting to such tactics.

[johnalex]

Hi dr john - just wondered whether you mark each cell with pencil 'alternatives' as recommended by DT's MM and other 'experts'? I have tried this but find it counter-productive and extremely un-aesthetic, like sweet wrappers in a maze. I reckon the clearer the grid, the easier it is to recognize pairs and patterns. However, I frequently reach the point you describe, then either solve it by brute force (pencil & rubber) or abandon the puzzle on the basis that no guesses are allowed!

Try the News of The World Sudoku on Sunday which is tougher than most. Also you might like to check out this web site and download the e-Book which you can enjoy on-screen and off-line, or print it into an A4 binder. (I paid US$17 which was �10 last December).

www.howtosolveeverysudokupuzzle.com

Good Luck, ja

[bongerman]

The Times are very good. All can be solved without guessing, although fairly complicated methods are required for tough and fiendish puzzles. I find that sometimes guessing can be the quicker option! Various sudoku chatrooms exist where people explain the harder rules if you want to improve your sudokuing abilities.

[cheaton]

I do the Times Sudoku every day. One trick is to write the missing numbers alongside each row or column. Sometimes you can then see that there is only one answer bec. 4 out of the 5 numbers (or 3 out of 4 etc) are 'spoken for'. By trying each missing number in the row individually AND as a group, you should get the answer.

You should never have to guess (altho' I suppose people do) but as Dr John says, you can guess wrongly unless you have 2 copies!. It is easier to show someone though than have to explain on an AB post!

And I'm still waiting to win the champagne despite sending it in on 9 days out of 10!

[jamspanner]

Go to http://www.divingforbananas.com/sudoku/assista nt.php and you will never guess again.
Guessing is completely unecessary and makes the puzzle worthless. If you use only logic, there is only one answer

[dr john]

Thanks all, esp those who suggested online resources. I'll try feeding my next "unsolvable" puzzle into them + hope to avoid guessing in future!