A counterpart of the Borel-Cantelli lemma
The general part of the Borel-Cantelli lemma says that for any sequence of events (An) defined on a probability space (Ω, Σ,P), the divergence of ΣnP(An) is necessary forP(Ani.o.) to be one (see e.g. [1]). The sufficient direction is confined to the case where the Anare independent. This paper provides a simple counterpart of this lemma in the sense that the independence condition is replaced byfor some. We will see that this property of (An) may frequently be assumed without loss of generality. We also disclose a useful duality which allows straightforward conclusions without selecting independent sequences. A simple random walk example and a new result in the theory ofϕ-branching processes will show the tractability of the method.