Need help with a Sudoku

[youtalkfunny]

Just tell me how to get the next number.

After two days of looking at it, I give up.

(EDIT to add bold lines, to make the grid more clear.)

[youtalkfunny]

This puzzle is the current one (10-20-06) at:

http://puzzles.usatoday.com/sudoku/

[TimM]

I don't even do Sudoku, but...

Is there a rule that there can only be one solution?

In the upper right box, for the last two squares, you can either go 5 8 or 8 5.

If you go 5 8, that forces most of the right side:



Trouble is, that if all of that is in the actual solution, then the 5 8 and 8 5 in the third and ninth rows could be reversed and that would also be a solution.

If you go 8 5 in that upper right box though, you are no longer forced to put the 5 in the 5th row 9th column. You can put it in the 4th row 7th column. Then you won't have an ambiguous solution.



I would work off this. My guess is if you do it the first way you will run into a dead end.

[youtalkfunny]

[ QUOTE ]
I don't even do Sudoku, but...

[/ QUOTE ]

When you do one, you're not supposed to fire in a guess, and see if it turns out right. You're supposed to be able to logically deduce what goes where.

Like you, I think the answer lies in those right-side columns.

[TimM]

[ QUOTE ]
When you do one, you're not supposed to fire in a guess, and see if it turns out right. You're supposed to be able to logically deduce what goes where.

[/ QUOTE ]

I'm not sure I even see the distinction you are trying to make here.

What is logical deduction anyway? Don't you start with a hyposthesis (educated guess)?

Would it be logical to say that if you put a 5 in the 5th row, 9th column, you can't ever have a unique solution, therefore you must put the 5 in the 4th row 7th column?

[youtalkfunny]

[ QUOTE ]
[ QUOTE ]
When you do one, you're not supposed to fire in a guess, and see if it turns out right. You're supposed to be able to logically deduce what goes where.

[/ QUOTE ]

I'm not sure I even see the distinction you are trying to make here.

[/ QUOTE ]

The last number I plugged in was the 1 in the 9th column. I deduced that that square must be a 1 because:

--That column needed a 1;
--I couldn't put it in the 5th row (that row has a 1)
--I couldn't put it in the lower-right box (that box has a 1)
--The only place in the 9th column I can place a 1 is the 3rd row, so I'll put it there.

That's how the thought process is supposed to work. You shouldn't have to plug in a dozen numbers before finding out you've headed down the wrong trail.

[youtalkfunny]

Here's the same puzzle, this time with my notes included.

Green numbers were given;
black numbers were filled in by me;
red numbers are my notes, noting all possible entries.

[TimM]

[ QUOTE ]
You shouldn't have to plug in a dozen numbers before finding out you've headed down the wrong trail.

[/ QUOTE ]

I'm not trying to be difficult here, I'm finding the philosophy of this whole thing interesting.

So wouldn't this the distinction between an easy problem and a hard one? You're saying there should be no look ahead at all in Sudoku, ever? If there can be, why couldn't the amount of look ahead needed sometimes be more than you can do in your head?

[amplify]

Just so you know, I ran it through a solver and [mild spoiler in white]

<font color="white">it said it had to guess (and there may be more than one solution) </font>

[TimM]

[ QUOTE ]
Here's the same puzzle, this time with my notes included.

Green numbers were given;
black numbers were filled in by me;
red numbers are my notes, noting all possible entries.



[/ QUOTE ]

Oh now it's easy, assuming you've made no mistakes so far. If you put the 5 in 5th row 9th column, you must put one in 6th row 2nd column, and then one in the 9th row 1st column, leaving no place for a 5 in the lower right box. Therefore, 5 in the 4th row 7th column.