BUG-Lite

BUG-Lite is a small-scale cousin of the BUG+1. The full Bivalue Universal Grave looks at the whole grid; BUG-Lite finds a partial pattern - a handful of cells whose two-candidate structure, if left as is, would let the puzzle finish two different ways. Because a fair Sudoku has one solution, that pattern must be broken, and breaking it hands you an elimination.

The idea

Look for a closed group of cells in which two digits alternate so that each of the digits appears exactly twice in every row, column and box the group touches. Such a group is a miniature "grave": you could swap the two digits all the way round it and satisfy every rule, giving a second solution. Since only one solution is allowed, at least one cell in the group must actually carry a different digit. Wherever a group cell has an extra candidate, the true solution has to use it there - so the two pattern digits can be removed from that cell, collapsing the grave.

A worked example

Six cells across three boxes each reduce to {4,6}, arranged so 4 and 6 each appear twice in every row, column and box they share - a swappable ring. One of those cells also holds a 1. If the puzzle is unique, that 1 cannot be idle: the solution must place it there. So 4 and 6 leave that cell, breaking the ring and usually leaving a single behind elsewhere.

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A balanced {4,6} ring - each digit twice per row, column and box. Swapping would give a second solution, so the extra 1 (top-left) is forced, clearing 4 and 6 there and collapsing the grave.

Step by step

  1. Find a partial bivalue pattern where two digits each appear twice per unit it touches.
  2. Confirm the swap would give a valid second solution.
  3. Locate the extra candidate in a pattern cell.
  4. Eliminate the two pattern digits there to break the grave.

Common mistakes

  • Not truly balanced. Each digit must appear exactly twice in every unit the pattern touches, or the swap is not free.
  • Variant grids. Cages and diagonals can forbid the swap, so keep BUG-Lite to standard, uniquely-solved puzzles.
  • No extra candidate. A perfectly balanced pattern with no extras signals a flawed puzzle, not an elimination.

Which Sudoku types is this best for?

The same logic applies across variants, but it pays off more in some than others.

Great fit Classic Sudoku A handy uniqueness shortcut on guaranteed-unique Classic grids, beyond the full BUG. → Limited Killer Sudoku Cage constraints can void the deadly pattern, so uniqueness logic is unsafe. → Limited Sudoku X The diagonals disrupt the bivalue pattern the technique relies on. → Works Samurai Sudoku Applies within each of the five standard subgrids. →

Where to practise this

This technique starts paying off at Expert, which is where puzzles first stop yielding to anything simpler. Reading about a pattern and spotting it on a live grid are different skills - the second one only comes from playing.

Play Expert Sudoku About Expert Sudoku

Related techniques

BUG+1 - Bi-Value Universal Grave Technique When all cells but one are bi-value, the extra candidate in that one cell must be the solution, or the puzzle would have two answers. X-Wing Sudoku Technique Explained One digit confined to the same two columns across two rows (or vice versa) - eliminate it from the rest of those lines. XY-Wing Sudoku Technique (Y-Wing) Explained A pivot cell with two candidates links two pincer cells; the digit they share is removed from any cell both pincers see. Swordfish Sudoku Technique Explained One digit confined to the same three columns across three rows (or vice versa) - eliminate it from the rest of those columns.
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