good point, marty, on the geometric vs. arithmetic.

>I don't know if there's a word for hybrid series like that.

me too, but the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, ...) is an example of a sequence described recursively by a pair of formulas:
FIBO(n) = FIBO(n-1) + FIBO(n-2) for n>2
FIBO(1) = FIBO(2) = 1