21305491 has 4 divisors (see below), whose sum is σ = 21315112. Its totient is φ = 21295872.

The previous prime is 21305483. The next prime is 21305497. The reversal of 21305491 is 19450312.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21305491 - 2³ = 21305483 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21305497) 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, 376 + ... + 6538.

It is an arithmetic number, because the mean of its divisors is an integer number (5328778).

Almost surely, 2^21305491 is an apocalyptic number.

21305491 is a deficient number, since it is larger than the sum of its proper divisors (9621).

21305491 is an equidigital number, since it uses as much as digits as its factorization.

21305491 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9620.

The product of its (nonzero) digits is 1080, while the sum is 25.

The square root of 21305491 is about 4615.7871484721. The cubic root of 21305491 is about 277.2238022396.

The spelling of 21305491 in words is "twenty-one million, three hundred five thousand, four hundred ninety-one".