Small Hadamard Matrices
A Hadamard matrix is a square matrix with all entries equal to or , with its rows mutually orthogonal. Half of the corresponding pairs of entries in any two rows have the same sign and half have the opposite sign.[more]
It is conjectured that a Hadamard matrix of order exists for all integers divisible by 4. However, many orders are unsolved, including 668, 716 and 892.[less]
 N. J. A. Sloane. "A Library of Hadamard Matrices." (Nov 11, 2021) neilsloane.com/hadamard.