20721 has 4 divisors (see below), whose sum is σ = 27632. Its totient is φ = 13812.

The previous prime is 20719. The next prime is 20731. The reversal of 20721 is 12702.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 20721 - 2^{1} = 20719 is a prime.

It is a D-number.

It is a Duffinian number.

It is a Curzon number.

It is a nialpdrome in base 12.

It is a junction number, because it is equal to *n*+sod(*n*) for *n* = 20697 and 20706.

It is not an unprimeable number, because it can be changed into a prime (20731) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3451 + ... + 3456.

It is an arithmetic number, because the mean of its divisors is an integer number (6908).

2^{20721} is an apocalyptic number.

It is an amenable number.

20721 is a deficient number, since it is larger than the sum of its proper divisors (6911).

20721 is an equidigital number, since it uses as much as digits as its factorization.

20721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6910.

The product of its (nonzero) digits is 28, while the sum is 12.

The square root of 20721 is about 143.9479072443. The cubic root of 20721 is about 27.4665157196.

It can be divided in two parts, 20 and 721, that added together give a triangular number (741 = T_{38}).

The spelling of 20721 in words is "twenty thousand, seven hundred twenty-one", and thus it is an iban number.

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