Recursion

Which of these functions utilize recursion?

1. fun f1(a : Int, b : Int) : Int = a + b
   

2. fun f2(n: Int): Int {
      if (n == 0 || n == 1) return 1
      return n * f2(n - 1)
   }
   

3. fun f3(n: Int): Int {
      var f = 1
      for (i in 1..n)
          f *= i
      return f
   }
   

4. fun f4(n : Int) {
      if(n < 1) return
      print(n)
      f4(n-1)
   }