DigiNum is an integer divisible by both the sum and product of its own
digits. Find a series of 3 DigiNums a, b, c, with none in the
intervals, such that they are an AP (Arithmetic Progression).
This means that c - b
= b - a.
The first term, a, must be between 10 and 50,000.
Example solution: 12, 24, 36 are all DigiNums, they are in AP, and the only DigiNum between 12 and 36 is 24.