Similarly, find a smallest-possible set of clues whose unique answer is (28b).
56. [47] What is the largest number of clues in a minimal sudoku puzzle?
57. [22] Every sudoku solution has at most 27 horizontal trios and 27 vertical trios, namely the 3-digit sets that appear within a single row or column of a box. For example, (28a) has nine horizontal trios {1, 2, 3}, {2, 3, 4}, ..., {9, 1, 2} and three vertical trios {1, 4, 7}, {2, 5, 8}, {3, 6, 9}; (28b) has just three of each. The solution to (29a) has 26 horizontal trios and 23 vertical trios; {3, 6, 8} occurs once horizontally, twice vertically.
Let T be the 27 trios {{A, B, C} | A ∈ {1, 2, 3}, B ∈ {4, 5, 6}, C ∈ {7, 8, 9}}. Find all sudoku solutions whose horizontal trios and vertical trios ...
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