Hi,

I have not a real questions, just some thoughts about Backdoors (Guessing a number, getting the Sudoku below SE 1.5) and wanna know your opinion. Well first of all, guessing must be allowed, when we allow Unique Rectangles. If a guess is right AND we know there is only one solution, we dont need to prove, that the other one is false.

If u consider SE ranking it turned out, that most Sudokus below 7.0 have at least one or two bivalue Backdoors. Thats huge, when you think about it. Most 6.x Sudokus dont have many bivalue cells, so the chance is very high that u find a backdoor, just by trying a few numbers.

Now here is my problem: A friend of mine and I, doing many Sudokus. He gives a [ed: expletive deleted] about theory. All he knows are Subsets, Intersections, x-Wing. He just solves normal "newspaper" Sudokus.

I told him, he cannot solve my 6.6 puzzles. But he did several times, and he did even faster then me. One puzzle for example it took me 2 hours to find all needed Turbot Fish, bivalue x-cycles and so on.

He just got to a dead end and tried to guess some bivalue cells. Thats how he solved it under half an hour. I thought this was just a coincidence, but it turned out, that he could solve every puzzle by that so far and most puzzles he did faster.

So why do I learn so much theory, if it is so easy even for Noobs?