'Premier' sudoku site????

Advanced methods and approaches for solving Sudoku puzzles

'Premier' sudoku site????

Postby stuartn » Tue Aug 23, 2005 3:14 pm

This site is 'claiming' to be the 'Worlds premier Sudoku Site'

http://www.number-logic.com/playgame.php?id=34

Now - forgive me if I'm wrong, but isn't the symmetry missing? - Could it be that this just another lot jumping on the bandwagon without realising the true nature and elegance of the exercise? - Please assure me that I'm wrong.

:D

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Postby Karyobin » Tue Aug 23, 2005 3:28 pm

I could do with that clarifying as well: what came first - symmetrical sudoku or non-so?

Karyobin (The World's Premier Maths Teacher - well, prove me wrong. Eight out of ten parents said their children preferred it).
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Postby noggi » Tue Aug 23, 2005 3:30 pm

The non symmetrical was first - found in America as Number Placing. The japanese made it symmetrical after a couple of years, and that was when it first "took off" according to Nikoli.
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Postby stuartn » Tue Aug 23, 2005 3:36 pm

Code: Select all
893752416
276143598
451869273
968437125
124695387
735218649
34 [27] 9 [27] 6851
68 [27] 5 [27] 1934
519384762


and one of the solutions to the one above is this. Hardly unique eh?

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Postby nj3h » Tue Aug 23, 2005 3:37 pm

I also thought "premier" was just a little too over the top.

The site has a daily puzzle. So what? With so many great programs out there (Pappocom, SadMan, Simple Sudoku) which can generate puzzles in a few seconds, why the excitement about getting one puzzle a day. Well, I guess the only advantage would be if one were on a 12-step program to break the sudoku addiction. What's the website address again?

Seriously, thanks to all who make this and the other forum a great asset. Those folks, and the developers mentioned above, are the real premier people.

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The 8/24 puzzle is beyond me, so far...

Postby Ross » Fri Aug 26, 2005 7:07 pm

I'm a sudoku newbie, but I can solve most Simple Soduku Extreme games. The 8/25 "Impossible" game on this site was fairly simple, but I am stymied by the 8/24 game. Simple Soduku says no hints available at this point:
Code: Select all
 *-----------*
 |.1.|..8|.4.|
 |.32|5..|...|
 |...|...|...|
 |---+---+---|
 |5..|..6|..7|
 |9..|...|..3|
 |8..|2.4|..9|
 |---+---+---|
 |...|...|...|
 |...|..7|98.|
 |.4.|81.|.6.|
 *-----------*


 *-----------*
 |.1.|..8|.4.|
 |.32|5..|..8|
 |...|...|...|
 |---+---+---|
 |523|986|417|
 |964|...|823|
 |871|234|659|
 |---+---+---|
 |...|...|...|
 |.56|..7|98.|
 |.4.|81.|.6.|
 *-----------*


{67}    {1}     {59}    {367}   {2679}  {8}     {235}   {4}     {256}   
{46}    {3}     {2}     {5}     {46}    {19}    {17}    {79}    {8}     
{467}   {89}    {589}   {13467} {24679} {1239}  {1235}  {39}    {1256} 
{5}     {2}     {3}     {9}     {8}     {6}     {4}     {1}     {7}     
{9}     {6}     {4}     {17}    {57}    {15}    {8}     {2}     {3}     
{8}     {7}     {1}     {2}     {3}     {4}     {6}     {5}     {9}     
{123}   {89}    {789}   {346}   {24569} {2359}  {12357} {37}    {1245} 
{123}   {5}     {6}     {34}    {24}    {7}     {9}     {8}     {124}   
{23}    {4}     {79}    {8}     {1}     {2359}  {2357}  {6}     {25}   
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Postby Sue De Coq » Fri Aug 26, 2005 10:28 pm

HINT: Consider the chain in the 9s: r2c6-r9c6-r9c3-r1c3-r1c5.
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Postby Ross » Fri Aug 26, 2005 11:11 pm

Thanks, but what technique produces that chain?
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Postby simes » Fri Aug 26, 2005 11:17 pm

Karyobin wrote:Karyobin (The World's Premier Maths Teacher - well, prove me wrong. Eight out of ten parents said their children preferred it).

Eight out of ten parents said their children preferred proving you wrong?
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Postby 737 driver » Sat Aug 27, 2005 2:32 am

Ross,

Great question. With a little computing power, a Nishio-guess of "what happens if I put a 9 in r2c6... okay, don't do that" takes less time than considering what chain to look at. Granted, I don't have much basis beyond intuition for going to that cel, but is there any greater basis to know that the productive chain to examine is the one in 9's starting there? After all, if you consider several chains before finding one that yields useful information, is that particularly different from considering the same number of Nishio-guesses in order to find the one that leads to a solution?

IF the answer to your question is that one can know a priori where to start looking to find the productive forcing chain, I'll admit that I'm off base... but if it takes as much "trail & error" searching for a useful chain as it takes looking for a useful elimination of candidates by Nishio, then I'm not sure how the forcing chains is any more an elegant approach.

Or even, all that different, for that matter.

As I said, excellent question!
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Postby Jeff » Sat Aug 27, 2005 2:52 am

737 driver wrote:but if it takes as much "trail & error" searching for a useful chain as it takes looking for a useful elimination of candidates by Nishio


Agreed. The process of finding the right set of chains can be T&E, but it is a different type of T&E that I enjoy doing. Personally, I am not interested in just getting the solution, because it will always have 1 to 9 in all its rows, columns and boxes. I enjoy the process that reveals the numbers one by one logically without bifurcation in my own definition.:)
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Postby Sue De Coq » Sat Aug 27, 2005 5:41 am

The aforemention chain forms a pattern that is very similar to the Turbot Fish described by nick70. (I'm not sure whether it's exactly a Turbot Fish as the two non-consecutive strong sides, i.e. r9c6-r9c3 and r1c3-r1c5, aren't connected by a box with just two candidates, as described in the original post on the subject. However, the underlying logical justification is almost identical). So, to find a chain such as this, simply use a slightly-extended form of whatever algorithm you use to find the Turbot Fish/Swordfish/X-Wing patterns. I claim that my tactic should be considered legitimate (i.e. not T&E) if Turbot Fish, Swordfish and X-Wing are all considered legitimate.

Now that I've re-read nick70, I see that he prefers a shorter many-valued chain to a longer single-valued chain, in which case he might like:

r7c5=9 => r7c2<>9 => r7c2=8 => r3c2<>8 => r3c2=9 => r1c3<>9 => r1c5=9, which has just four legs. (Since r7c5=9 leads to a contradiction, it's eliminated, which forces the 9 in Column 5 into Box 2, so 9 is no longer a candidate for r2c6).
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Postby Karyobin » Sat Aug 27, 2005 11:19 am

simes wrote:
Karyobin wrote:Karyobin (The World's Premier Maths Teacher - well, prove me wrong. Eight out of ten parents said their children preferred it).

Eight out of ten parents said their children preferred proving you wrong?


Quite right simes, I apologise for my ambiguous grammar. As I said, I'm the World's Premier Maths Teacher, not English.

Oh, by the way - BRING IT ON!!!
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Postby george-no1 » Sat Aug 27, 2005 11:23 am

What's the 'it' you're asking people to 'bring on', if you don't mind me asking?

G:)
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Postby Karyobin » Sat Aug 27, 2005 11:32 am

Dunno, W. G. Grace says it. A lot. I take it to imply hunger for competition and a desire to trounce the convicts. In the best possible spirit, of course.
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