The indicator [random variable] of the event A, often written 1_A, is the random variable that equals unity if A occurs, and zero if A does not occur. The expected value of the indicator of A is the probability of A, P(A), and the standard error of the indicator of A is (P(A)×(1−P(A))^½. The sum

1_A + 1_B + 1_C + …

of the indicators of a collection of events {A, B, C, …} counts how many of the events {A, B, C, …} occur in a given trial. The product of the indicators of a collection of events is the indicator of the intersection of the events (the product equals one if and only if all of indicators equal one). The maximum of the indicators of a collection of events is the indicator of the union of the events (the maximum equals one if any of the indicators equals one).