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Positive periodic solutions for a nonlinear functional differential equation. (English) Zbl 1220.34090
Summary: Sufficient conditions are obtained for the existence of at least two positive periodic solutions of Nicholson’s blowflies model
$x'(t)=-a(t)x(t)+p(t)x^m(t-\tau(t))e^{-\gamma(t)x^n(t-\tau(t))}.$
The Leggett-Williams multiple fixed point theorem is used to prove our results.

##### MSC:
 34K13 Periodic solutions to functional-differential equations 47N20 Applications of operator theory to differential and integral equations
##### Keywords:
periodic solution; nonnegative solution