An important aspect of cryptography with matrices is that given N ×N matrix P over a field F find a class of matrices G over F such that the associated doubly circulant matrix Gc is singular in order that the equation AGB = P in circulant matrices A, B has infinitely many solutions. The aim of this note is to present such a class of matrices G. We also present a direct method of finding the inverse of a symmetric circulant matrix of order n, a b b ... b where a + (n − 1)b = 0.
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