Monkeymore Sudoku

XY-Chain

المستوى 10 · Brown Belt

المتطلب السابق: Multi-Digit Chains

Overview

XY-Chain links bivalue cells (cells with exactly 2 candidates) through shared candidates. Each consecutive pair of cells shares a candidate, creating alternating strong and weak inferences that force eliminations.

How It Works

  1. Find bivalue cells that share a candidate with neighboring bivalue cells.
  2. Build a chain where consecutive cells share one candidate, alternating which is the shared digit.
  3. The first and last cells must share a common candidate on their strong links.
  4. Eliminate that common candidate from cells seeing both endpoints.

Example

Found an XY-Chain of 3 bivalue cells from r1c7 to r2c3.

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Tips & Patterns

  • XY-Chains extend XY-Wing to longer chains.
  • Every cell in the chain must have exactly 2 candidates.
  • The key insight: the common candidate at the endpoints must be in at least one endpoint, so cells seeing both can't have it.