Strassen's 7 multiplications vs 8: how matrix multiplication hits O(n^2.81)

Franc0Fernand0 · x · 2026-10-14

Splitting matrix multiplication into blocks doesn't speed it up — removing one multiplication does. Naive multiplication is O(n³); divide-and-conquer still needs 8 sub-multiplications per recursion level, so it stays O(n³). Strassen's algorithm recombines the same blocks with only 7 multiplications per step, dropping complexity to O(n^2.81) — a small constant win that matters greatly at large sizes. The attached image shows the seven product formulas from Strassen's 1969 paper.

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