摘要

This paper deals with the existence and multiplicity of periodic solutions to delay differential equations of the form (z) over dot(t) = -f(z(t - 1)) - f(z(t - 2)) - ... - f(z(t - 2n + 1)) where z is an element of R-N, f is an element of C(R-N,R-N). By using the S-1 pseudo geometrical index theory in the critical point theory, some known results for Kaplan-Yorke type differential delay equations are generalized to higher dimensional case. As a result, the Kaplan-Yorke's conjecture is proved to be true in the case of higher dimensional systems.