20021 has 2 divisors, whose sum is σ = 20022. Its totient is φ = 20020.

The previous prime is 20011. The next prime is 20023. The reversal of 20021 is 12002.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12100 + 7921 = 110^2 + 89^2 .

It is a cyclic number.

It is not a de Polignac number, because 20021 - 2¹⁴ = 3637 is a prime.

Together with 20023, it forms a pair of twin primes.

It is a Chen prime.

It is a nialpdrome in base 14.

It is a junction number, because it is equal to n+sod(n) for n = 19987 and 20014.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20023) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10010 + 10011.

It is an arithmetic number, because the mean of its divisors is an integer number (10011).

2²⁰⁰²¹ is an apocalyptic number.

It is an amenable number.

20021 is a deficient number, since it is larger than the sum of its proper divisors (1).

20021 is an equidigital number, since it uses as much as digits as its factorization.

20021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4, while the sum is 5.

The square root of 20021 is about 141.4955829699. The cubic root of 20021 is about 27.1536733044.

Adding to 20021 its reverse (12002), we get a palindrome (32023).

Multiplying 20021 by its reverse (12002), we get a palindrome (240292042).

The spelling of 20021 in words is "twenty thousand, twenty-one", and thus it is an iban number.