CageDoku2-cell cage with sum 4 in Killer Sudoku: possible combinations
By Andrew Boyko, developer of CageDoku · Updated
A 2-cell Killer Sudoku cage that adds up to 4 has exactly one possible digit set: 1-3.
Digits in this cage
| Combinations | 1 3 |
|---|---|
| Always in the cage | 1, 3 |
| Never in the cage | 2, 4, 5, 6, 7, 8, 9 |
| Candidates for every cell | 1, 3 |
How to use it
Write the candidates 1, 3 in every cell of the cage and cross out everything else. Because 1, 3 are in every combination, those digits are certain to sit somewhere in the cage. If the cage lies inside a single row, column or box, any digit that must be in the cage can be removed from the rest of that region. Also watch rows, columns and boxes that already contain some of these digits: a cell that sees a digit cannot hold it, and a combination that needs it can be ruled out.
One combination to memorise
This is a unique combination: 1, 3. You do not know which cell holds which digit, but you know exactly which digits are used, so every other candidate can go.
Neighbouring sums
Sum 3 (1 combination) · Sum 5 (2 combinations). See the full 2-cell chart or use the calculator to add must-have or excluded digits.
Frequently asked questions
Which digits can a 2-cell cage with sum 4 contain?
Only 1, 3. The digits 2, 4, 5, 6, 7, 8, 9 never appear in this cage.
How many combinations make 4 with 2 different digits?
Exactly one: 1-3.
Which digits are always in a 2-cell cage that sums to 4?
1, 3 appear in every combination.
Practice it in CageDoku
Free, ad-free Killer Sudoku and seven more variants. Hints name the technique and highlight the cells, so you learn why a digit goes where it goes.