21021 has 24 divisors (see below), whose sum is σ = 38304. Its totient is φ = 10080.

The previous prime is 21019. The next prime is 21023. The reversal of 21021 is 12012.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21019) and next prime (21023).

It is not a de Polignac number, because 21021 - 2¹ = 21019 is a prime.

It is a Curzon number.

21021 is a lucky number.

It is a nialpdrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21023) 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 23 ways as a sum of consecutive naturals, for example, 1611 + ... + 1623.

It is an arithmetic number, because the mean of its divisors is an integer number (1596).

2²¹⁰²¹ is an apocalyptic number.

21021 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21021 is a deficient number, since it is larger than the sum of its proper divisors (17283).

21021 is a wasteful number, since it uses less digits than its factorization.

21021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 41 (or 34 counting only the distinct ones).

The product of its (nonzero) digits is 4, while the sum is 6.

The square root of 21021 is about 144.9862062405. The cubic root of 21021 is about 27.5984351140.

Adding to 21021 its reverse (12012), we get a palindrome (33033).

Subtracting from 21021 its reverse (12012), we obtain a palindrome (9009).

It can be divided in two parts, 210 and 21, that added together give a triangular number (231 = T₂₁).

The spelling of 21021 in words is "twenty-one thousand, twenty-one", and thus it is an iban number.