Variations on the Two Envelopes Problem
  There are many papers written on the Two Envelopes Problem that usually study some of its variations. In this paper we will study and compare the most significant variations of the problem. We will see the correct decisions for each player and we will show the mathematics that supports them. We will point out some common mistakes in these calculations and we will explain why they are incorrect. Whenever an amount of money is revealed to the players in some variation, we will make our calculations based on the revealed amount, something that is not achieved in other papers. 
Panagiotis Tsikogiannopoulos
math.HO physics.data-an