"A picture of a 428x428 Hadamard matrix, i.e. a matrix with orthogonal columns and all entries equal to +1 (white pixels) or -1 (black pixels). This example was discovered by H. Kharaghani and B. Tayfeh-Rezaie in 2004, and was the first of size 428x428. It is not known if there is a 668x668 Hadamard matrix, though it is conjectured that examples of size 4n x 4n exist for all n."