an integer n of length at least two digits. If n is even, divide by 2;
if it's odd, add it to the nearest prime number greater than itself. If
the result of either operation is just a single digit integer, the
series terminates. If not, go around again.
Example: 13, 30, 15, 32, 16, 8
What's the longest chain required to complete any sequence starting with a 2-digit integer? Or to complete a 3- or 4-digit integer sequence?