• 20 can be written using four 4's:
• Deleting all the even digits from 220 = 1048576 we obtain a prime (157).
20 is nontrivially palindromic in base 3 and base 9.
20 is an esthetic number in base 6, because in such base its adjacent digits differ by 1.
20 is an admirable number.
It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
It is a magnanimous number.
It is a O'Halloran number.
20 is an undulating number in base 3.
20 is a nontrivial repdigit in base 9.
It is a plaindrome in base 7, base 8, base 9, base 11, base 12, base 13, base 14, base 15 and base 16.
It is a nialpdrome in base 4, base 5, base 6, base 9 and base 10.
It is a zygodrome in base 9.
It is a self number, because there is not a number n which added to its sum of digits gives 20.
It is a congruent number.
It is a panconsummate number.
It is the 4-th tetrahedral number.
A polygon with 20 sides can be constructed with ruler and compass.
It is an amenable number.
20 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
20 is a wasteful number, since it uses less digits than its factorization.
20 is an evil number, because the sum of its binary digits is even.
The square root of 20 is about 4.4721359550. The cubic root of 20 is about 2.7144176166.