“…In a recent paper Cabada and Tojo [6] studied, by means of methods and results present in [4,5], the first order operator u ′ (t) + ω u(−t) coupled with periodic boundary value conditions, describing the eigenvalues of the operator and providing the expression of the associated Green's function in the non-resonant case. One motivation for studying this particular problem is that differential equations with reflection of the argument have seen growing interest along the years, see for example the papers [1,3,6,10,11,21,[23][24][25][26]31] and references therein. In [6] the authors provide the range of values of the real parameter ω for which the Green's function has constant sign and apply these results to prove the existence of constant sign solutions for the nonlinear periodic problem with reflection of the argument u ′ (t) = h(t, u(t), u(−t)), t ∈ [−T, T ], u(−T ) = u(T ).…”