BUG+1

Level 4 · Yellow Belt (Orange Stripe)

Strategy:

Requires:

Overview

BUG (Bivalue Universal Grave) is a state where all cells have exactly two candidates and each appears twice per unit - creating multiple solutions. BUG+1 has one cell with three candidates; the extra candidate must be the solution.

How It Works

Find a near-BUG state: all cells have 2 candidates except one cell with 3.
In the 3-candidate cell, identify which candidate appears an odd number of times in its row, column, and box.
That candidate (appearing 3 times instead of 2) must be the solution for this cell.

Example

Look for BUG+1.

Sudoku grid showing the initial puzzle state for Look for BUG+1.

1

BUG+1

Examine all unsolved cells - every cell has exactly 2 candidates except one.

Sudoku grid illustrating step 1: BUG+1 — Examine all unsolved cells - every cell has exactly 2 candidates except one.

2

BUG+1

Cell r1c1 has 3 candidates. This is a 'BUG+1' pattern.

Sudoku grid illustrating step 2: BUG+1 — Cell r1c1 has 3 candidates. This is a 'BUG+1' pattern.

3

BUG+1

If r1c1 were reduced to 2 candidates, we'd have a 'Bivalue Universal Grave' with multiple solutions.

Sudoku grid illustrating step 3: BUG+1 — If r1c1 were reduced to 2 candidates, we'd have a 'Bivalue Universal Grave' with multiple solutions.

4

BUG+1

A valid puzzle has one solution, so r1c1 must be 9 (the 'extra' digit appearing 3 times in a row/column/box).

Sudoku grid illustrating step 4: BUG+1 — A valid puzzle has one solution, so r1c1 must be 9 (the 'extra' digit appearing 3 times in a row/column/box).

5

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

BUG+1 is easy to spot when you're near the end of a puzzle.
Count candidate occurrences - the "odd one out" is the answer.
This technique assumes puzzle uniqueness.