21391091 has 2 divisors, whose sum is σ = 21391092. Its totient is φ = 21391090.

The previous prime is 21391057. The next prime is 21391129. The reversal of 21391091 is 19019312.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21391091 - 2²² = 17196787 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21391091.

It is not a weakly prime, because it can be changed into another prime (21391021) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10695545 + 10695546.

It is an arithmetic number, because the mean of its divisors is an integer number (10695546).

Almost surely, 2^21391091 is an apocalyptic number.

21391091 is a deficient number, since it is larger than the sum of its proper divisors (1).

21391091 is an equidigital number, since it uses as much as digits as its factorization.

21391091 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 486, while the sum is 26.

The square root of 21391091 is about 4625.0503781040. The cubic root of 21391091 is about 277.5945775073.

The spelling of 21391091 in words is "twenty-one million, three hundred ninety-one thousand, ninety-one".