input: Write a function to determine if a given number is abundant.
  assert is_abundant(12)==True
  assert is_abundant(13)==False
  assert is_abundant(9)==False
output: def is_abundant(n):
      if n < 1:
          return False
      divisors_sum = sum(i for i in range(1, n) if n % i == 0)
      return divisors_sum > n