Note on Williamson Matrices of Orders \(25\) and \(37\)

All Williamson matrices in this Note are symmetric circulants. Eight non-equivalent sets of Williamson matrices of order \(25\) are known. They were discovered by Williamson (\(2\) sets), Baumert and Hall (\(2\) sets), and Sawade (\(4\) sets). Sawade carried out a complete search and reported that there are exactly eight non-equivalent such sets of matrices. Subsequently, this was confirmed by Koukouvinos and Kounias. It is surprising that we have found two more such sets. Hence, there are ten non-equivalent sets of Williamson matrices of order \(25\).

Only three non-equivalent sets of Williamson matrices of order \(37\) were known so far. One of them was discovered by each of Williamson, Turyn, and Yamada. We have found one more such set.