• 20 can be written using four 4's:
• Deleting all the even digits from 220 = 1048576 we obtain a prime (157).
20 has 6 divisors (see below), whose sum is σ = 42. Its totient is φ = 8.
The previous prime is 19. The next prime is 23. The reversal of 20 is 2.
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 a nontrivial binomial coefficient, being equal to C(6, 3).
It can be written as a sum of positive squares in only one way, i.e., 16 + 4 = 4^2 + 2^2 .
It is a sliding number, since 20 = 10 + 10 and 1/10 + 1/10 = 0.20.
20 is an admirable number.
It is a Harshad number since it is a multiple of its sum of digits (2).
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.
It is a pernicious number, because its binary representation contains a prime number (2) of ones.
A polygon with 20 sides can be constructed with ruler and compass.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2 + ... + 6.
It is an arithmetic number, because the mean of its divisors is an integer number (7).
It is a pronic number, being equal to 4×5.
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 20, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (21).
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.
With its successor (21) it forms an eRAP, since the sums of their prime factors are consecutive (9 and 10).
20 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 9 (or 7 counting only the distinct ones).
The product of its (nonzero) digits is 2, while the sum is 2.
The square root of 20 is about 4.4721359550. The cubic root of 20 is about 2.7144176166.
The spelling of 20 in words is "twenty", and thus it is an aban number, an iban number, an oban number, and an uban number.
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