21305891 has 2 divisors, whose sum is σ = 21305892. Its totient is φ = 21305890.

The previous prime is 21305857. The next prime is 21305909. The reversal of 21305891 is 19850312.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21305891 - 2¹⁰ = 21304867 is a prime.

It is a Sophie Germain prime.

It is not a weakly prime, because it can be changed into another prime (21305191) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10652945 + 10652946.

It is an arithmetic number, because the mean of its divisors is an integer number (10652946).

Almost surely, 2^21305891 is an apocalyptic number.

21305891 is a deficient number, since it is larger than the sum of its proper divisors (1).

21305891 is an equidigital number, since it uses as much as digits as its factorization.

21305891 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2160, while the sum is 29.

The square root of 21305891 is about 4615.8304778230. The cubic root of 21305891 is about 277.2255371418.

The spelling of 21305891 in words is "twenty-one million, three hundred five thousand, eight hundred ninety-one".