>>Erdős found that for any primitive set, including infinite ones, that sum — the “Erdős sum” — is always finite. No matter what a primitive set might look like, its Erdős sum will always be less than or equal to some number. And so while that sum “looks, at least on the face of it, completely alien and vague,” Lichtman said, it’s in some ways “controlling some of the chaos of primitive sets,” making it the right measuring stick to use.

Finite? Surely there must be an operation done to each element of the set before summing to achieve a finite bound.