A recursive function is a function defined by a recursion and initial conditions. One example would be f(n+1) = f(n) + 1 for all n, and f(0) = 0. Of course, this function must be f(x) = x. But not all recursive functions can be written explicitly. Also, your definition of "recursive function" might not include functions that can be written explicitly. What specifically would you like to understand about recursive functions?