# Feedback and Questions

I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page
is where I try to answer them. Please feel free to drop me a note on the side of the page or try the Facebook
comment box. Or you can email me directly at andrew@str8ts.com.
On Holiday until 23rd, yay! Feedbacks answered as soon as I get back
**Post a Comment or Question here...**

## Thursday 27-May-2010

## ... by: Brian, Australia

In your Xwing solutions etc. you offer explanations such as (I quote)

"- Contradiction: When D7 is set to 4 the chain implies it cannot not be 4 - it can be removed"

The words "cannot not" be something indicate that it must be that something. You then say to remove it. I don't believe that a double negative is appropriate or correct in the explanation.

Please correct me if I am mistaken here.

Brilliant program anyway.

## Tuesday 18-May-2010

## ... by: Bob Kukla, Texas, USA

If you're looking for something else to do to improve it, you could add support for the arrow keys during the entry process. Most important one would be the down arrow but all directions would be valuable.

Keep up the good work!

Bob

## Wednesday 12-May-2010

## ... by: Julia, USA

## Sunday 9-May-2010

## ... by: David G. Stork, California

HIDDEN PAIR: 1/6 in row 6: F6 - 1/3/6 -> 1/6

## Saturday 1-May-2010

## ... by: Brad, Michigan

## Sunday 18-Apr-2010

## ... by: Fred, Belfast

## Friday 26-Mar-2010

## ... by: Mike, UK

The technique helped to solve the puzzle - just that it is not one of the listed strategies

## Monday 22-Mar-2010

## ... by: Charlie, Nebraska

## Friday 12-Mar-2010

## ... by: Bill Richter, Chicago

The obvious reason to use mathematical elegance rather than pattern spotting as the criterion is to avoid shouting match about who spotted patterns and who asked the question "If I remove/add a number here - what is the consequence?". Furthermore, there is no reason why we should care if folks ask the consequences of plugging in numbers. Chess players do this routinely (If I move my rook, they'll move the bishop, then I can take the pawn), and no one scolds them for it. Our actual opposition to plugging in numbers is that is the inelegant proofs that folks post as a result of plugging in numbers. We want to rule out solutions using inelegant proofs like "I plugged 6 into B5, and 30 moves later, I have two 5s in box 8, so 6 can't go there!" We want simple proofs that we can draw nice pictures of. If we drew a picture of this inelegant proof, we could make the 6 in B5 blue, and candidates turned off red, and andidates turned on green, but we would also need subscripts to show at which time the candidates were turned on or off. That's much more complicated than the pictures your solver draws, and so not elegant.

But let me expand on your point that the line between 'trial and error' vs determinism is blurry. On a hard puzzle, folks often prove a statement X is true by using an OR/NAND chains of the form

A1 OR B1 NAND A2 OR B2 ... An OR Bn,

for statements Ai & Bi, where X NAND A1 and Bn NAND X are true. But how does one find such a useful chain? If the OR statements Ai OR Bi are, say, of the ALS/AAHS form, there may be 10s of thousands of such chains. Nobody is going to form all possible such chains and then see if any of them work. Let's say they start with the first OR statements A1 OR B1. They have to glue another OR statements A2 OR B2 to it, meaning that B1 NAND A2 is true. But that's the same thing as saying that B1 => B2, and that the deduction B1 => B2 is of the ALS/AAHS form, meaning that it's a relatively simple deduction. So we can get into truly ludicrous discussions of whether the solver spotted the ALS/AAHS pattern which gave the OR statement A2 OR B2, or whether the solver _really_ noticed that the deduction B1 => B2 was simple. There's also the ludicrous discussion of whether the solver was looking for a chain which proved X, but that is perhaps a meaningful question, even though we can never know what the solver was _really_ thinking.

So I contend we should just concentrate on the mathematical proofs of eliminations, and leave off considerations of the solver's mental states, which we can't study, can't describe precisely, and shouldn't care about. As examples of elegant mathematical solutions of mine in which I explicitly violated the blurry rules, see my solution of Mepham's puzzle Unsolvable #14 (http://www.sudoku.org.uk/SudokuThread.asp?fid=4&sid=10290&p1=3&p2=14) and Unsolvable #30 (http://www.sudoku.org.uk/SudokuThread.asp?fid=4&sid=10234&p1=4&p2=14), which I believe you wrote. It was a nice puzzle, and Steve Kurzhals liked my solution

## Friday 12-Mar-2010

## ... by: sss, usa

## Saturday 6-Mar-2010

## ... by: Oliver Buckley, London

Load Sudoku: CLICK TO LOADAt this stage in solving the puzzle, your Solution Count records three viable solutions. Your main program eliminates two of these by referring to Uniqueness (Type 1), however this is incorrect. The uniqueness referred to cannot eliminate any solution, as both the other two solutions are equally valid and in fact clues from other grids in the Samurai confirm that one of the solutions your Uniqueness strategy eliminates, is in fact the correct one. I suspect there is a bug in the Uniqueness process.