15)What is a closed-form formula? 16)What is the procedure for proving a statement by mathematical induction? 17)For defining palindromes, why is it not enough to use just the empty string as a base case? 18)Describe in your own words the language defined by this recursive definition: = @ | | @ = aab | aa b 19)Write a recursive definition for the set of positive odd numbers. 20)Explain how you would design a method, returning boolean, that determines if a string passed as a parameter satisfies this recursive definition. = = a | b | a | b