19. Write a recursive mathematical definition for computing 2" for a positive integer n. 20. Which of the following statements are true? 1. Any recursive function can be converted into a nonrecursive function. 2. A recursive function takes more time and memory to execute than a nonrecursive function 3. Recursive functions are always simpler than nonrecursive functions. 4. There is always a selection statement in a recursive function to check whether a base case is reached