““Oresme's genius was to make a new series which was definitely smaller than the harmonic series. He took the list of all unit fractions, and for any of them which did not have a power of two as a denominator, he replaced it with a smaller fraction which did. As all these new fractions were either the same or smaller, the total of this new series had therefore to be smaller than the sum of the harmonic series. But when Oresme grouped these fractions into runs, each of which added up to 1/2, he was left with a sum of an infinite sequence of 1/2s, which definitely diverges. This meant in turn that the greater harmonic series must also diverge. Oresme had proved that a sequence of ever-decreasing numbers could still be divergent. (His proof was lost for a while, and the same result was independently rediscovered in the 1600s.)””
