REAs Essentials provide quick and easy access to critical information in a variety of different fields, ranging from the most basic to the most advanced. As its name implies, these concise, comprehensive study guides summarize the essentials of the field covered. Essentials are helpful when preparing for exams, doing homework and will remain a lasting reference source for students, teachers, and professionals. Probability includes basic concepts, counting methods, conditional probability and independence, random variables, sums of random variables, countably infinite random variables, continuous random variables, special discrete probability models, special continuous probability distributions, generating functions, and Markov chains.