In the present paper we are
concerned with the existence of T-periodic solutions for the differential equation
ẋ(t) = f(t,x(t)), t ∈R, where f is a continuous time dependent T-periodic tangent
vector field defined on an n-dimensional differentiable manifold M possibly with
boundary. We prove that if the Euler characteristic of the average vector field
w(p) = (1∕T)∫0Tf(t,p)dt is defined and nonzero and if all the possible orbits of the
parametrized equation ẋ(t) = λf(t,x(t)), t ∈ R and λ ∈ (0,1], lie in a compact set
and do not hit the boundary of M, then the given equation admits a T-periodic
solution.