I do not know why for the life of me why I cannot solve simple recurrence relations. Could someone please help me with the following:

I have developed some code which searches through a list of integers and retrieves the N largest ones (where N can be as big, but no bigger than, the size of the list).

I have to determine the complexity (Big-Oh) of this function, but I cannot seem to prove the recurrence. I know this runs on O(n) time, but I need to be able to derive that from a recurrence formula. So here's the formula:

I know this is ridiculously simple, but I can't figure it out.

Oh and:

F(0) = 0

Any help would be greatly appreciated