> "For example, rather than counting how many numbers are in a set, they might do the following: For every number n in the set, plug it into the expression 1/(n

> "For example, rather than counting how many numbers are in a set, they might do the following: For every number n in the set, plug it into the expression 1/(n log n), then add up all the results. The size of the set {2, 3, 55}, for instance, becomes 1/(2 log 2) + 1/(3 log 3) + 1/(55 log 55). Erdős found that for any primitive set, including infinite ones, that sum — the “Erdős sum” — is always finite." Read the paragraph just above it.