Sudoku Strategy Help Please!

[ronaldx]

I recently started to look at Nikoli's records of how other people have solved sudoku.

What I've noticed is that some of the very fast solvers are using techniques that I definitely don't have! They are placing numbers that I can't place, or see how they can be placed, until (much) later in the puzzle - and they're doing it fast

I'd like to talk you through some of these...

One example comes from the Sudoku on Nov 14
https://member.nikoli.com/play/about/6657.html
Looking at speedy Hideaki Jo's solution - click on his time, 2:06
(Unfortunately I think you need to be logged into Nikoli to see this - I would prefer to use a public site if anyone knows of one that provides puzzle walkthrus)

I can see how the first two 7s are placed, but then the next 2? I understand the 2 is in this column but why is it in that row: it doesn't seem to immediately follow... but H.Jo seems confident in placing it after a couple of seconds and is correct about it.
This doesn't seem to be the natural next placement, I think it shows that H.Jo has a different technique to mine.

Any help?

This might be a trivial miss on my part... but there are several more examples of this
If I get some interest I will add more to this thread

[ronaldx]

Update: I see he is really placing the 6 first, which makes more sense, but I'm still not sure how he did that so fast.

Let's skip a little later on... when he places the 4 and the 1 in the top left block. Why that way around???

[detuned]

I guess I count as one of nikoli's quickest - I don't use anything that isn't well documented on scanraid. I'll have a look at what H.Jo did in that example in the next day or two...

[detuned]

Yeah...who am I kidding next couple of days haha.

Ok what he's done is basically a law of leftovers. In the bottom left 3x3 box, the 267 are forced into 3 positions. The 7 places trivially, and the 2 and 6 are easily resolved by a the crossing 6 on the 8th row.

Similar deal with the 1 and 4 in the top left 3x3 box. You are guaranteed to have the 384 in the 3 empty squares of the first 6 of the 2nd column. Notice the crossing 8 and 3 in the 3rd row.

I wouldn't feel too bad about this. A similar technique utterly stumped the grand finalists at the Times champs in September, and even had me stumped for a little while whilst simul-solving

[ronaldx]

The placement of the 4 is extremely interesting, thanks. I could have looked at that for days.