21391211 has 2 divisors, whose sum is σ = 21391212. Its totient is φ = 21391210.

The previous prime is 21391193. The next prime is 21391213. The reversal of 21391211 is 11219312.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21391211 - 2¹⁸ = 21129067 is a prime.

It is a Sophie Germain prime.

Together with 21391213, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21391213) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10695605 + 10695606.

It is an arithmetic number, because the mean of its divisors is an integer number (10695606).

Almost surely, 2^21391211 is an apocalyptic number.

21391211 is a deficient number, since it is larger than the sum of its proper divisors (1).

21391211 is an equidigital number, since it uses as much as digits as its factorization.

21391211 is an evil number, because the sum of its binary digits is even.

The product of its digits is 108, while the sum is 20.

The square root of 21391211 is about 4625.0633509175. The cubic root of 21391211 is about 277.5950965908.

It can be divided in two parts, 21 and 391211, that multiplied together give a triangular number (8215431 = T₄₀₅₃).

The spelling of 21391211 in words is "twenty-one million, three hundred ninety-one thousand, two hundred eleven".