20921 has 2 divisors, whose sum is σ = 20922. Its totient is φ = 20920.

The previous prime is 20903. The next prime is 20929. The reversal of 20921 is 12902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19321 + 1600 = 139^2 + 40^2 .

It is a cyclic number.

It is not a de Polignac number, because 20921 - 2⁶ = 20857 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is a plaindrome in base 11.

It is a junction number, because it is equal to n+sod(n) for n = 20896 and 20905.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 20921.

It is not a weakly prime, because it can be changed into another prime (20929) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10460 + 10461.

It is an arithmetic number, because the mean of its divisors is an integer number (10461).

2²⁰⁹²¹ is an apocalyptic number.

It is an amenable number.

20921 is a deficient number, since it is larger than the sum of its proper divisors (1).

20921 is an equidigital number, since it uses as much as digits as its factorization.

20921 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 36, while the sum is 14.

The square root of 20921 is about 144.6409347315. The cubic root of 20921 is about 27.5546022573.

The spelling of 20921 in words is "twenty thousand, nine hundred twenty-one".