Philippe De Vitry , a 14th CenturyFrenchMusicologist , defined a harmonic number as an integer that is not divisible by any prime other than 2 or 3. In 1342 , the RabbiGersonides , a contemporary of de Vitry, showed that there are only four pairs of such numbers that differ by one. The Abc Conjecture concerns a generalization of Gersonides results.