Assuming that the incidence of each letter in a passage is independent of the incidence of the other letters, and provided m is much larger than 26, the probability that a sequence of length m will contain all 26 letters is approximately
Given that the occurrence of each letter in a passage is independent and m is significantly greater than 26, we can approximate the likelihood that a sequence of length m will encompass all 26 letters by considering the total number of possible sequences and the probability of each individual letter appearing.