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A harmonic prime(sequence A092101 in the OEIS ) is a prime number that divides the numerators of exactly three harmonic numbers.
Specifically, a harmonic prime p is always a factor of the numerators of the partial harmonic sums at positions p − 1, p2 − p, and p2 − 1.
For example, the numerators of the fractions given by , , and are 25, 55835135, and 1347822955, each of which is divisible by 5.
All prime numbers greater than 5 can also be found at those three indices, but many also appear at other indices. It is conjectured that there are infinitely many harmonic primes. [1]