The New Sudoku Players' Forum

[sydneydenizen]

Code: Select all

8 ? 6 | 4 9 5 | ? ? 1
? 3 ? | 8 7 6 | 5 9 ?
5 9 ? | ? ? ? | 8 6 ?
=====================
4 5 3 | 1 6 8 | 2 7 9
? 6 9 | ? 4 ? | ? 5 8
7 ? 8 | 5 ? 9 | 4 ? 6
=====================
3 4 ? | 6 ? ? | 9 8 5
6 8 ? | 9 5 3 | ? 4 ?
9 ? 5 | ? 8 4 | 6 ? 3

I often get to this stage where there are no more obvious next steps (well, obvious to me). I can usually only finish them by making a guess/assumption and following through to success or failure.

Are there any tips/tricks to use at this stage?

Thanks in advance!

[Animator]

You need to guess in order to complete the grid...

This can mean two things:
a) it's not a Pappocom puzzle
b) you missed a clue

[sydneydenizen]

a) it's not a Pappocom puzzle

You might be right. This puzzle is out of the Sydney Morning Herald and they don't seem to be listed on this website.

b) you missed a clue

I'm not sure what other clues there are that I would have missed? The numbers are exactly as I have posted them.

[Animator]

Then you need to guess. There is no other way to complete it

[tso]

No guess is required.

Code: Select all

  8   2-7   6  |  4    9    5  | 3-7  2-3   1
 1-2   3    .  |  8    7    6  |  5    9   2-4
  5    9    .  |  .    .    .  |  8    6   247
  -------------+---------------+-------------
  4    5    3  |  1    6    8  |  2    7    9
 1-2   6    9  |  .    4    .  | 1-3   5    8
  7   1-2   8  |  5    .    9  |  4   1-3   6
  -------------+---------------+-------------
  3    4    .  |  6    .    .  |  9    8    5
  6    8    .  |  9    5    3  |  .    4    .
  9    .    5  |  .    8    4  |  6    .    3


Though Pappocom rates this as "arguably unfair", it can be solved by FCP (Forcing Chains Proof) in any number of places. For example:

Consider r5c7 and its relationship to r2c1:

1) r5c7 must be 1 or 3.
2) If r5c7=1 => r5c1=2 => r2c1=1
3) If r5c7=3 => r1c7=7 => r1c2=2 => r2c1=1

Since all possibilites for r5c7 lead to r2c1=1, then r2c1 MUST be 1. The rest of the puzzle solves quickly.

See http://forum.enjoysudoku.com/viewtopic.php?t=692

[datprogrammer]

Consider r5c7 and its relationship to r2c1:

1) r5c7 must be 1 or 3.
2) If r5c7=1 => r5c1=2 => r2c1=1
3) If r5c7=3 => r1c7=7 => r1c2=2 => r2c1=1

Si
See http://forum.enjoysudoku.com/viewtopic.php?t=692

Nonsense!!!

r5c7=1 forces r6c2 to be 2
if r6c2 is 2 then r5c1 must be 1 and not 2

[simes]

Nonsense!!!

r5c7=1 forces r6c2 to be 2
if r6c2 is 2 then r5c1 must be 1 and not 2

eh? TSO's reasoning looks fine to me!

r5c7=1 forces r6c2 to be 2

How so?

[tso]

Consider r5c7 and its relationship to r2c1:

1) r5c7 must be 1 or 3.
2) If r5c7=1 => r5c1=2 => r2c1=1
3) If r5c7=3 => r1c7=7 => r1c2=2 => r2c1=1

Si
See http://forum.enjoysudoku.com/viewtopic.php?t=692

Nonsense!!!

r5c7=1 forces r6c2 to be 2
if r6c2 is 2 then r5c1 must be 1 and not 2

You've made a mistake. As it turns out, in the completed puzzle, r5c7=1 and r5c1=2. Though it IS possible for something false to imply something true, it is NOT possible for something true to imply something false. Therefore there can be NO line of reasoning in which r5c7=1 (true) forces r5c1=1 (false).

[datprogrammer]

I'm hanging my head in shame - it seems I'm no longer able to understand cartesean co-ordinates.

I'll slink away slowly into the sunset and hope that one day I willbe forgiven for being so stupid!

My only possible defense is that I had just woken up and was yet to have my caffeine hit!

My apologies - I'm not usually so dense - honest!!!