21001 has 2 divisors, whose sum is σ = 21002. Its totient is φ = 21000.

The previous prime is 20983. The next prime is 21011. The reversal of 21001 is 10012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15376 + 5625 = 124^2 + 75^2 .

It is a cyclic number.

It is not a de Polignac number, because 21001 - 2⁷ = 20873 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21011) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10500 + 10501.

It is an arithmetic number, because the mean of its divisors is an integer number (10501).

2²¹⁰⁰¹ is an apocalyptic number.

It is an amenable number.

21001 is a deficient number, since it is larger than the sum of its proper divisors (1).

21001 is an equidigital number, since it uses as much as digits as its factorization.

21001 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2, while the sum is 4.

The square root of 21001 is about 144.9172177486. The cubic root of 21001 is about 27.5896796813.

Adding to 21001 its reverse (10012), we get a palindrome (31013).

Multiplying 21001 by its reverse (10012), we get a palindrome (210262012).

It can be divided in two parts, 2 and 1001, that multiplied together give a palindrome (2002).

The spelling of 21001 in words is "twenty-one thousand, one", and thus it is an iban number.