Sudoku-YAdvanced
When to assume—and when a branch is actually disproved
Twenty successful-looking steps do not prove the starting digit. Ask whether every step was forced and whether the necessary alternatives were covered.
Practise on a board ↓LOOK · TRY · UNDERSTAND
Try it and see why
No conflict yet. Is it proved?
Row A needs 2, 5, 7, with only A1/A4/A8 empty and complete notes. Try A1=2.
Our own partial teaching board, not a full puzzle. Large digits are placed values; small digits are candidates. Dots mean the cell is not described here. Complete candidate lists are part of the stated scenario, not calculated from the dots.
Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate
Follow the steps. What can we conclude?
See the reasoning
A1=2 forces A4=5, leaving A8 with no digit. Reject A1=2; with the complete notes given, A1 must be 7. That does not solve the whole puzzle.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: HoDoKu — Last Resort
Try familiar deductions first
Check singles, pairs and locked candidates you understand. If you remain stuck, a two-candidate cell makes a manageable experiment. This is our suggested practice sequence, not a rule requiring assumptions after a fixed time.
What disproves an assumption?
A logically derived contradiction can rule it out: a cell has no candidate, or a missing digit has nowhere to go in a unit. If you made an additional guess along the way, a dead end may disprove only that later guess. It does not automatically refute the original premise.
Example: A={3,6}
If A=3 leads through forced deductions to a contradiction, eliminate 3 and keep 6. If both branches still proceed without a conflict, the question remains open.
Try before revealing
A=6 survives 20 steps. Is it proven?
Show explanation
No. You may not have reached the contradiction yet. You need further evidence, such as eliminating the alternatives, or completing a grid that satisfies every rule and starting clue when the puzzle is known to have one solution.
A result shared by all alternatives
If an exhaustive set of alternatives forces the same conclusion, that conclusion can hold without selecting a branch. This idea appears in forcing chains; the detailed rules of every chain type are beyond this introduction.
In Sudoku-Y
Independent what-if cases start from the same board, without nested layers. Switch, reset or leave them to recover your main board. “No conflicts found yet” is a current check, not proof of correctness. Read the reasoning and search limits attached to a hint.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.