BUG+1

Level 5 · Orange Belt

Strategy:

BUG+1

Requires:

Naked Pair

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

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

Example

Starting position

Look for BUG+1.

1

All cells bivalue except one

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

2

r2c6 is the BUG+1 cell

Cell r2c6 has 3 candidates. This is a 'BUG+1' pattern.

3

Two candidates would make a BUG

If r2c6 were reduced to two candidates, the puzzle would become a Bivalue Universal Grave with multiple solutions.

4

Place 9 at r2c6

A valid puzzle has one solution, so r2c6 must be 9, the extra digit that appears three times in a row, column, or box.

5

Result

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.