Sudoku Puzzles

just about to start

thought I'd skip to the tough first - just to see if I could be first - but alas

3:54

Forgot to set the timer doh!

I am not first but I am here.

c'mon Brick from Brizzy, whom are you kidding?
it took me longer to put all the possibilities in

to george from romania
george you're just envious;)
Brick does it without setting up (read yday)

15:00 why same bird?or is it diff.

even i do it w/o any settingup.i tried with the red blinkers.then too i took a solid seven mts. to complete the grid.kudos to brick if he is so fast

I'm having trouble posting this comment. There's an interesting article about the mathematics of sudoku. URL is
http://www.americanscientist.org/template/AssetDetail/assetid/48550?&print=yes

In discussing the distinction between trial-and-error and logic, says: 'Backtracking itself can be viewed as a logical operation; it supplies a proof by contradiction. '

19:23

Here's my full solution. Usual disclaimer about typos -- I'm sure that it's not the simplest, but it got the job done.

Part 1:

1. In column-i, 1 must be in i8 or i9; eliminate from i4, i5 & i6
2. In column-f, 1 must be in f2 or f3; eliminate from f4 & f5
3. In column-g, 2 must be in g8 or g9; eliminate from g4 & g6

Part 2:

4. a1 & a2 must = 2 or 8; elimiante from rest of column-a
5. Easy elimination: e8=3, c7=3, g3=3
6. e9 & f9 must = 6 or 7; eliminate from rest of row-9
7. In column-d; 8 must be in d7 or d8; eliminate from d5 & d6
8. In row-3, 9 must be in a3 or b3; eliminate from c1 & c2
9. In column-d, 5, 8 & 9 must be in d7, d8 or d9; eliminate from rest of column

Part 3:

10. In column-f, 1, 5 & 6 must be in f1, f2 or f3; eliminate from rest of column & 3x3
11. East elimination: f9=7, e9=6
12. In row-8, 6 must be in a8 or c8; eliminate from g8 & i8
13. Easy elimination: g6=6 (6 eliminated from all other cells in column), a5=6, c8=6, c4=9, More...

Part 4:

14. In column-c, 7 must be in c1 or c2; eliminate from c6
15. b5 & b6 must = 5 or 8; eliminate 1, 4 & 7 from these cells
16. Easy elimination: b4=7 (7 eliminated from all other cells in 3x3), e6=7, e5=5, b5=8, b6=5
17. Looking at e2 = 2 or 4, can conclude that i4=3; proof: More...

Hey Howard........Those American scientists are justs cheats who are too big-headed to admit it !

Part 5:

18. Easy elimination: a6=3
19. Looking at d5 = 1 or 4, can conclude that i6=8; proof if d5=1, then h5=4 & i6=8; if d5=4, then e4=2, h6=2 & i6=8
20. h1 & h2 must = 5 or 8; eliminate 4, 6 & 7 from these cells
21. Easy elimination: h7=7, i7=6, h3=6, f3=1, b3=4, c1=7, i1=4, d1=2, ... easy eliminations to end

Howard & Rob... having read the article, I don't think that this person is making that statement absolutely. Rather, he is saying that the argument can be made that it's a type of logic -- albeit not that type most refer to when we say solving the puzzle by logic alone.

He does go so far as to say that deduction such a what I used in step 19 is not 'trial and error.'

Actually, he doesn't say that outright... he just says, 'David Eppstein of the University of California at Irvine has formulated some extremely subtle Sudoku rules, which capture the kind of information gleaned from a backtracking analysis, yet work in a forward-looking, nonspeculative mode.'

All that work to get the same dumb bird???

We need better pictures on tough!

cool bird 4:45 with an astonishingly large amount of help

and scientist and proud

1:22! I think the timer must be wrong cos that's no way I finished it in 1:22...

The article and some of the mild controversey in these comment pages has to do with terminology. The article says: 'If you solve by logic, you never write a number into a cell until you can prove that only that number can appear in that position.' And by that definition, your step 19 is logical -- i6 can only be 8.

But then the article says: 'For the logic-only strategy to work, a puzzle must have a quality of progressivism: At every stage in the solution, there must be at least one cell whose value can be determined unambiguously. ... The backtracking protocol dispenses with progressivism: When you reach a More...

In my opinion, TMs derivation is entirely logical. No cell is filled in until the value has been proven correct. But one does reach a point where every vacant cell has at least two candidates. And alternatives have to be explored; the choices of which alternatives to explore is arbitrary.

About the step 19: it is still logical since one has to find a chain within the grid and not only within a single column, row or block. It's not a random choise, neither a 'trial'. When you find that chain, whether you read it one way, you get a number, whether you read it in the other way, you still get the same number (I mean clockwise or anticlockwise since we are talking about oddgons)

I did it, without mistakes or checking moves, but it took me a long time(37 minutes). Used several hidden triples and 2 forcing chains. A ton of work to get the same pic. LOL. Does anyone have any tips on selecting the cells to try for the forcing chains?

Sorry Mike, no tips on selecting the cells. That's the only 'trial and error' I use when solving. If I'm lucky, I pick a cell that works on the first try. Sometimes, I need to test 4-5 different cells before I find one that works.

4:46 My time was a bit faster than the Hard level :)

I think that is a beautiful picture of the bird and feeder.

12;04 Great time for me. Picture is a treat since the hummingbirds have gone south and will not be seeing them again until spring...May in the states.

Having a problem with step #15, why must 'b5 & b6 must = 5 or 8;' I have 8 for a possibility in b4, where did I miss that elimination, or what is the thought process for b5 & b6 being 5 or 8?

i did the easy then skiped 2 the tough, didn't take me long

Nevermind, after posting my comment/question I noticed my puzzle was erased, so decided to go back through solving it your way. Thanks! For all the work

Hi everyone! to easy I go. did ok today time? don't time myself.