One of the most widely known unitary matrices is the DFT matrix, which also has a well known fast multiplication algorithm $(O(n \ \textrm{log} \ n))$, the Fast Fourier Transform. Do all unitary matrices share the property of having a (not necessarily known) fast multiplication algorithm? Or is this unique to the DFT?
2026-03-25 19:01:13.1774465273
Do all unitary matrices have fast multiplication algorithms?
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