Counting, Symmetries and Equivalence Classes of Sudoku Grids

Sudoku is a widely popular puzzle whose complete grids have been enumerated computationally: there are approximately $6.67 \times 10^{21}$ of them and, up to symmetry and renaming of digits, $5,472,730,538$ essentially different ones.

The classical enumeration reduces the count to $44$ equivalence classes of the first band through a chain of ad hoc reductions, leaving the number $44$ without any apparent structural explanation.

We present an alternative derivation of these $44$ classes, in which they arise as isomorphism classes of unordered triples (multisets) of column partitions under relabeling, a single invariant that replaces the original chain of reductions.

This invariant makes it possible to apply Burnside's Lemma by hand: we recover $44$, together with two intermediate stages of the classical reduction, through a closed derivation requiring no computational enumeration.