Periodic solutions for a scalar integro-differential equation (Q1886950)

The paper is concerned with sufficient conditions for the existence of periodic solutions to scalar integro-differential equations with unbounded delay of the form \[ y'(t)=a(t)y(t)+\int_{-\infty}^t k(t,r)g(r,y(r))\,dr. \] The methodology used is based on the Krasnoselskij fixed point theorem. Following preliminaries in which the basic ideas and known theory are summarised, the authors prove their main theorems which are then used to establish the existence of periodic solutions for various biological model equations, as described in the final sections of the paper.