MathSciNet bibliographic data MR1144718 (92m:34152) 34K15 Chen, Yong Shao The existence of periodic solutions of the equation $x'(t)=-f(x(t),x(t-\tau))$$x'(t)=-f(x(t),x(t-\tau))$. J. Math. Anal. Appl. 163 (1992), no. 1, 227–237. Article

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