The sequence: 1, 21, 321, 4321, etc is very simple to generate. It is the sum of k(10^(k-1)) for all k between 1 and n, with the results of n=1, 2, 3, 4 shown above. It only gets slightly messier when n is in double digits, for instance the 14th number in the sequence is: 14320987654321, and the 28th number is 30987654320987654320987654321.

It was conjectured that no number in this sequence is prime, but that turns out to be false.

Can you find the first counterexample? Just to warn you this cannot be done without a computer.