We investigated the signal function of vocal imitation of contact calls in orange-fronted conures (OFCs; Aratinga canicularis) in Costa Rica. OFCs live in dynamic social systems with frequent flock fusions and fissions. Exchanges of contact calls precede these flock changes. During call exchanges, the similarity between the contact calls of different individuals may either increase (converge) o...

This paper concerns a duality between conditionally convergent real series and permutations of their indices. It is widely known that if one is given a series E ^ which is conditionally convergent but not absolutely convergent, then there is a permutation n such that £«< # £#*(*)• Various authors have studied extensions of this result of Riemann (for example, Smith [8], Steinitz [9], Threlfall ...