Pentagon numbers
Pentagonal numbers are generated by the formula, Pn=n(3*n-1)/2. The
first ten pentagonal numbers are:

1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...

It can be seen that P(4) + P(7) = 22 + 70 = 92 = P(8). However, their
difference, 70 - 22 = 48, is not pentagonal.

Find the pair of pentagonal numbers, P(j) and P(k), for which their sum
and difference are pentagonal and D = |P(k) - P(j)| is minimised; what
is the value of D?