21315391 has 2 divisors, whose sum is σ = 21315392. Its totient is φ = 21315390.

The previous prime is 21315389. The next prime is 21315421. The reversal of 21315391 is 19351312.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21315391 - 2¹ = 21315389 is a prime.

Together with 21315389, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21315341) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10657695 + 10657696.

It is an arithmetic number, because the mean of its divisors is an integer number (10657696).

Almost surely, 2^21315391 is an apocalyptic number.

21315391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21315391 is an equidigital number, since it uses as much as digits as its factorization.

21315391 is an evil number, because the sum of its binary digits is even.

The product of its digits is 810, while the sum is 25.

The square root of 21315391 is about 4616.8594303920. The cubic root of 21315391 is about 277.2667346876.

The spelling of 21315391 in words is "twenty-one million, three hundred fifteen thousand, three hundred ninety-one".