21491023 has 4 divisors (see below), whose sum is σ = 21896568. Its totient is φ = 21085480.

The previous prime is 21491021. The next prime is 21491027. The reversal of 21491023 is 32019412.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21491023 - 2¹ = 21491021 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21490985 and 21491003.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21491021) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 202693 + ... + 202798.

It is an arithmetic number, because the mean of its divisors is an integer number (5474142).

Almost surely, 2^21491023 is an apocalyptic number.

21491023 is a deficient number, since it is larger than the sum of its proper divisors (405545).

21491023 is an equidigital number, since it uses as much as digits as its factorization.

21491023 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 405544.

The product of its (nonzero) digits is 432, while the sum is 22.

The square root of 21491023 is about 4635.8411318767. The cubic root of 21491023 is about 278.0261823416.

The spelling of 21491023 in words is "twenty-one million, four hundred ninety-one thousand, twenty-three".