Can someone please explain and help me understand these 2 problems?

5. (5 pts) Give a recursive definition with initial condition(s) for a. -an- -V5 where n = 1,2,3, recwse: anNS The function fin) = n! where n =0,1,2, ease: Fo)l b. c. S = {4, 8, 12, 16, 20, ) recurs: (5 pts) Each type of machine part made in a factory is stamped with a code of the form "letter-digit-digit", where the digits cannot be repeated. If 10,000 parts are made, then how many must have the same code stamped on them? 5 6. 26 lo 1ogoo 2