this equation being the characteristic equation of a real binary linear substitution of determinant 1. If A is not a multiple of Tiri, it is clear that the two solutions (2) are linearly independent. If X is a multiple of Tiri, it depends on the elementary divisors of that binary substitution whether or not the general solution of (1) is free of secular terms. The equation (1) determined by the given periodic function p(t) is said to be of the stable type if every solution x(t) remains bounded as t-> + oo, i. e., if the elementary divisors are simple and X lies on the imaginary axis of the A-plane. Since from (3) ei-X A -H(A2_1)*, it follows that (4) -1?A 1 is necessary and that (5) -1 < A < 1