Factors / Factors of 21

Factors of 21

1, 3, 7, 21

21 breaks into 2 pairs, tested up to √21 ≈ 4.6; each pair multiplies back to 21 exactly. 21's full pair list is (1, 21), (3, 7), for 4 divisors in total. 21's divisors other than 21 itself sum to just 11, 10 below 21: the common deficient case. 21 is odd, digit sum 3 for 21; small divisors of 21 include 3, 7. 21 has 4 divisors; 20 has 6 and 22 has 4, so 20 wins here. 21 divided by its largest divisor, 7, gives 3.