The case notes
A unimodular line keeps one remainder
Digits on a unimodular line all belong to the same modulo-three class: {1,4,7}, {2,5,8}, or {3,6,9}. Watch a worked example, then try the idea in a puzzle.
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Unimodular means every digit belongs to one remainder group. The groups are one-four-seven, two-five-eight, and three-six-nine.
1 and 4 choose {1,4,7}
This line begins with one and four. Both belong to one-four-seven, so the final cell must come from that group too.
1 and 4 used → 7
One and four already appear in the row. The only choice left is seven.
Same class, wherever the rule applies
Some puzzles apply this rule to whole diagonals. Read the puzzle rule to see which lines must stay in one class.
Read the transcript
Unimodular means every digit belongs to one remainder group. The groups are one-four-seven, two-five-eight, and three-six-nine. This line begins with one and four. Both belong to one-four-seven, so the final cell must come from that group too. One and four already appear in the row. The only choice left is seven. Some puzzles apply this rule to whole diagonals. Read the puzzle rule to see which lines must stay in one class.
