1. Sethi 4.6. 2. Determine the fibonnaci value for an arbitrary index, but which only computes each value in the series once. (e.g. If you the input is 6, and f(1) and f(2) are both known to be 1, you may only computer f(3) once, and you may only compute f(4) once.... N.B. this is easy to do iteratively, please do it recursively.)