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A Hadamard matrix is an n x n matrix, all of whose entries
are ∓()1, such that every pair of rows and every pair of columns
differ in exactly n/2 places.
Two such matrices are considered equivalent if one can be transformed
into the other by performing row swaps, column swaps, row negations or
column negations.
The problem of deciding whether two Hadamard matrices are equivalent
is hard.
Magma contains several specialised routines for working with Hadamard
matrices; of special note is the introduction of a canonical form for
such matrices (based on Brendan McKay's nauty program or Adolfo
Piperno's Traces package).
This leads to a much faster equivalence algorithm than previously
available.
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