## tough sudoku collection

Post puzzles for others to solve here.

### tough sudoku collection

Greetings all

I've been working on a collection of very hard sudoku puzzles that I'd like to share. I've created a web page called "The Sudoku Puzzles from Hell!" It contains the puzzles, solution, and hints.

http://manifestmaster.com/sudoku-from-hell/

I welcome any suggestions, feedback, criticisms etc. Let me know what you think.

Enjoy!
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

Hi, i looked at some. Seems, they are all solvable with advanced basics (x-wing, xy-wing, skyscraper, fish, UR), not needing any chains (?).
Then nice for pencil&paper players, but easy for this thread.
eleven

Posts: 1945
Joined: 10 February 2008

### Re: tough sudoku collection

I did design this collection with pencil users in mind. I also have a collection of puzzles that require brute force for solution. I'd be glad to add a new link to those if you think there's interest in them.
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

Here's an example of a brute force puzzle I came up with

Code: Select all
`800002960000600400647090032900006071004000620560000049050060790006007200000800006`

At this point, I can't find anything other than brute force that works

Code: Select all
`'+-------------------------------+-------------------------------+-------------------------------+'| 8         13        135       | 13457     13457     2         | 9         6         57        |'| 123       1239      12359     | 6         13578     1358      | 4         158       578       |'| 6         4         7         | 15        9         158       | 158       3         2         |'+-------------------------------+-------------------------------+-------------------------------+'| 9         238       238       | 2345      23458     6         | 358       7         1         |'| 137       1378      4         | 1359      1358      1359      | 6         2         358       |'| 5         6         1238      | 1237      12378     138       | 38        4         9         |'+-------------------------------+-------------------------------+-------------------------------+'| 1234      5         1238      | 123       6         134       | 7         9         348       |'| 134       1389      6         | 1359      135       7         | 2         158       3458      |'| 12347     12379     1239      | 8         1235      13459     | 135       15        6         |'+-------------------------------+-------------------------------+-------------------------------+`

Brute force: r1c4 = 4, then you'll use it again r2c3 = 5

I'm still fairly new to this forum so I'll take any advice or suggestions. If this forum lives for these type of puzzles, I have lots of them to share
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

Code: Select all
`┌────────────────────┬────────────────────┬────────────────────┐│  478     14     78 │    3      2     19 │   79      6      5 ││                    │                    │                    ││  567     15      9 │  147      8     16 │    3    147      2 ││                    │                    │                    ││   67      2      3 │  149     67      5 │    8     14    149 │├────────────────────┼────────────────────┼────────────────────┤│    3      8      4 │    6      9      2 │    1      5      7 ││                    │                    │                    ││    9      7      5 │    8      1      4 │    2      3      6 ││                    │                    │                    ││    2      6      1 │    5      3      7 │    4      9      8 │├────────────────────┼────────────────────┼────────────────────┤│  578     59      6 │    2      4    139 │   79    178    139 ││                    │                    │                    ││    1     49     28 │   79     67    369 │    5     28    349 ││                    │                    │                    ││   47      3     27 │   19      5      8 │    6   1247    149 │└────────────────────┴────────────────────┴────────────────────┘`

This is puzzle #1

UR (14)r23c48 internal and external
1r9c4=7r2c8--(7=91)r1c76-r23c5=r9c4=> 1r9c4
Marty R.

Posts: 1508
Joined: 23 October 2012
Location: Rochester, New York, USA

### Re: tough sudoku collection

Marty R. wrote:
This is puzzle #1

UR (14)r23c48 internal and external
1r9c4=7r2c8--(7=91)r1c76-r23c5=r9c4=> 1r9c4

You lost me there. Not familiar with that puzzle, plus it has not possible solution. Did I miss something?
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

docjohn,

the game we are playing here is, that we start at the position, where no basic moves (singles, locked candidates, pairs, triples, quads) can be applied anymore.
The we try to find a single step, which solves the puzzle, leaving only singles to the end (stte).

E.g. your Puzzle 11 looks quite interesting:
Code: Select all
` +-------+-------+-------+ | . . . | . . . | . . 5 | | . . 9 | 4 2 . | . 6 8 | | . . 2 | . . . | . . . | +-------+-------+-------+ | . . . | . . . | . . 2 | | . . . | 8 5 . | 7 . . | | . 3 8 | 2 1 . | . . . | +-------+-------+-------+ | . 6 . | . . . | . 1 4 | | 1 . 3 | . . 4 | 2 . 7 | | 8 . . | . . 9 | . . . | +-------+-------+-------+`

After some pairs you come here:
Code: Select all
` *---------------------------------------------------------------* |  346    148    16   |  7    369   18    |  1349   2     5     | |  357    157    9    |  4    2     15    |  13     6     8     | |  3456   1458   2    |  36   369   158   |  1349   7     139   | |---------------------+-------------------+---------------------| |  567    157    57   |  9    4     36    |  138    38    2     | |  2      49     16   |  8    5     36    |  7      349   139   | |  49     3      8    |  2    1     7     |  56     459   69    | |---------------------+-------------------+---------------------| |  579    6      57   |  35   38    2     |  89     1     4     | |  1      59     3    |  56   68    4     |  2      89    7     | |  8      2      4    |  1    7     9     |  56     35    36    | *---------------------------------------------------------------*`

Instead of using skyscraper, ER and x-wing now, we try to find one (harder) step, which solves it.
Normally the step is a chain (often using ALS's - almost locked sets), an advanced pattern or a combination, like Marty showed above - wherever his puzzle is from.

Your brute force puzzle is much too hard here. It needs a couple of complex nets to solve it.
eleven

Posts: 1945
Joined: 10 February 2008

### Re: tough sudoku collection

You lost me there. Not familiar with that puzzle, plus it has not possible solution. Did I miss something?

What do I know? Maybe I missed something. I clicked the link in the original post, then clicked on puzzle #1
Marty R.

Posts: 1508
Joined: 23 October 2012
Location: Rochester, New York, USA

### Re: tough sudoku collection

I see another one there, but for docjohn:
The solution is correct:
To avoid a unique rectangle 14 in cells r23c48 - which would mean, that the puzzles has at least 2 solutions -, either r9c4=1 or r2c8=7. The chain shows, that r2c8=7 also implies r9c4=1. So it must be true, and only singles are left to solve.
eleven

Posts: 1945
Joined: 10 February 2008

### Re: tough sudoku collection

eleven wrote:I see another one there, but for docjohn:
The solution is correct:
To avoid a unique rectangle 14 in cells r23c48 - which would mean, that the puzzles has at least 2 solutions -, either r9c4=1 or r2c8=7. The chain shows, that r2c8=7 also implies r9c4=1. So it must be true, and only singles are left to solve.

I see that elimination and it makes sense. My confusion is that I thought Marty was looking at puzzle #1 from my site, but he's showing a different puzzle (which is insolvable)
Puzzle #1 on my site is this:

Code: Select all
`'+-------------------------------+-------------------------------+-------------------------------+'| 1267      124       1246      | 14789     1468      5         | 13679     13469     1467      |'| 9         145       1456      | 3         2         1467      | 1567      8         14567     |'| 3         8         1456      | 1479      146       14679     | 15679     2         14567     |'+-------------------------------+-------------------------------+-------------------------------+'| 12        6         7         | 5         13        13        | 4         139       8         |'| 18        9         1348      | 147       1346      2         | 1356      136       156       |'| 5         1234      1234      | 14        9         8         | 1236      7         126       |'+-------------------------------+-------------------------------+-------------------------------+'| 1268      125       125689    | 12489     1458      149       | 12678     146       3         |'| 4         123       1238      | 6         7         13        | 128       5         9         |'| 1268      7         1235689   | 12489     13458     1349      | 1268      146       1246      |'+-------------------------------+-------------------------------+-------------------------------+`
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

Code: Select all
`+-------------------------------+-------------------------------+-------------------------------+'| 1267      124       1246      | 14789     1468      5         | 13679     13469     1467      |'| 9         145       1456      | 3         2         1467      | 1567      8         14567     |'| 3         8         1456      | 1479      146       14679     | 15679     2         14567     |'+-------------------------------+-------------------------------+-------------------------------+'| 12        6         7         | 5         13        13        | 4         139       8         |'| 18        9         1348      | 147       1346      2         | 1356      136       156       |'| 5         1234      1234      | 14        9         8         | 1236      7         126       |'+-------------------------------+-------------------------------+-------------------------------+'| 1268      125       125689    | 12489     1458      149       | 12678     146       3         |'| 4         123       1238      | 6         7         13        | 128       5         9         |'| 1268      7         1235689   | 12489     13458     1349      | 1268      146       1246      |'+-------------------------------+-------------------------------+-------------------------------+`

to crack that puzzle open after some basic steps: its to this point
Code: Select all
`+--------------+---------------+--------------------+| 7   14   2   | 189   148  5  | 1369   1346  146   || 9   145  156 | 3     2    67 | (15)   8     1457  || 3   8    156 | (19)  14   67 | (159)  2     1457 |+--------------+---------------+--------------------+| 2   6    7   | 5     13   13 | 4      9     8     || 8   9    4   | 7     6    2  | 135    13    15    || 5   13   13  | 4     9    8  | 26     7     26    |+--------------+---------------+--------------------+| 16  25   9   | 28    58   4  | 7      16    3     || 4   23   8   | 6     7    13 | (12)   5     9     || 16  7    35  | (12)  35   9  | 8      146   146-2 |+--------------+---------------+--------------------+`

Almost Locked Set XZ-Rule: A=r39c4 {129}, B=r238c7 {1259}, X=9, Z=2 => r9c9<>2
then it collapses to singles to the end.

Code: Select all
`800002960000600400647090032900006071004000620560000049050060790006007200000800006`
this ones pretty difficult, ill look at cracking it when i have more free time
Some do, some teach, the rest look it up.

StrmCkr

Posts: 900
Joined: 05 September 2006

### Re: tough sudoku collection

see that elimination and it makes sense. My confusion is that I thought Marty was looking at puzzle #1 from my site, but he's showing a different puzzle (which is insolvable)
Puzzle #1 on my site is this:

I can see that I erred in that the puzzle I show Isn't puzzle #1 as I claimed. I don't know where it came from. But I don't understand why docJohn says it's unsolvable. I thought I showed a solution and that others confirmed the solution.

I just went to that site, clicked on puzzle #1, clicked Ecport on the left and got a 404 error.
Marty R.

Posts: 1508
Joined: 23 October 2012
Location: Rochester, New York, USA

### Re: tough sudoku collection

Marty R. wrote:I just went to that site, clicked on puzzle #1, clicked Export on the left and got a 404 error.

Oops!
Nice catch Marty. My bad. Its fixed now. I just added the export feature to all the puzzles today. thanks for the heads up
(I'll go back and look at the "unsolvable" puzzle again. Maybe its just me)
docjohn

Posts: 25
Joined: 07 January 2011

### Re: tough sudoku collection

Unsolvable to the limitation of their implemented technique set.
Some do, some teach, the rest look it up.

StrmCkr

Posts: 900
Joined: 05 September 2006

### Re: tough sudoku collection

docjohn,
Congratulations on your efforts. However, to pick up on one point in your definitions; what you call a strong link is actually a conjugate link. With a strong link it is possible for both the linked terms to be true. I haven't checked if this affects any of your other tutorial pages though. See the <AIC Primer Thread>

David PB
.
David P Bird
2010 Supporter

Posts: 1040
Joined: 16 September 2008
Location: Middle England

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