  Our main source of inspiration was a talk by Hendrik Lenstra on harmonic
numbers, which are numbers whose only prime factors are two or three.
Gersonides proved 675 years ago that one can be written as a difference of
harmonic numbers in only four ways: 2-1, 3-2, 4-3, and 9-8. We investigate
which numbers other than one can or cannot be written as a difference of
harmonic numbers and we look at their connection to the $abc$-conjecture. We
find that there are only eleven numbers less than 100 that cannot be written as
a difference of harmonic numbers (we call these $ndh$-numbers). The smallest
$ndh$-number is 41, which is also Euler's largest lucky number and is a very
interesting number. We then show there are infinitely many $ndh$-numbers, some
of which are the primes congruent to $41$ modulo $48$. For each Fermat or
Mersenne prime we either prove that it is an $ndh$-number or find all ways it
can be written as a difference of harmonic numbers. Finally, as suggested by
Lenstra in his talk, we interpret Gersonides' theorem as "The $abc$-conjecture
is true on the set of harmonic numbers" and we expand the set on which the
$abc$-conjecture is true by adding to the set of harmonic numbers the following
sets (one at a time): a finite set of $ndh$-numbers, the infinite set of primes
of the form $48k+41$, the set of Fermat primes, and the set of Mersenne primes.
