The probability that a discrete random variable equals any of its values is always non-negative (i.e., greater than or equal to zero). Also, the sum of the probabilities of all possible values of a discrete random variable is always equal to 1.
This means that if X is a discrete random variable that can take on the values x1, x2, x3, ..., xn, then the probability that X equals any one of these values is given by:
P(X = xi) ≥ 0 for all i = 1, 2, ..., n
And the sum of all these probabilities is:
P(X = x1) + P(X = x2) + P(X = x3) + ... + P(X = xn) = 1
This is known as the probability mass function (PMF) of the random variable X, and it provides a complete description of the probabilities of all possible values of X.