  Iterated Prisoner's Dilemma(IPD) is a well-known benchmark for studying the
long term behaviors of rational agents, such as how cooperation can emerge
among selfish and unrelated agents that need to co-exist over long term. Many
well-known strategies have been studied, from the simple tit-for-tat(TFT)
strategy made famous by Axelrod after his influential tournaments to more
involved ones like zero determinant strategies studied recently by Press and
Dyson. In this paper, following Press and Dyson, we consider one memory
probabilistic strategies. We consider that we call invincible strategies: a
strategy is invincible if it never loses against any other strategy in terms of
average payoff in the limit, if the limit exists. We show a strategy is
invincible for infinite repeated iterated prisoner's dilemma.
