It's like Fibonacci but starting with only one male rabbit, and adding a female in the second month. Rabbits are immortal, and male/female pairs produce one offspring every month. Numbers of males and females are kept equal as far as possible - that is, two pairs of rabbits will produce a male/female pair, and a single pair of rabbits will produce a male, unless an odd male already exists from an earlier month, in which case they will produce a female.

Produce a closed form expression for the number of rabbits after the nth month, and use this to compute the total number of rabbits after 5 years.