21511991 has 2 divisors, whose sum is σ = 21511992. Its totient is φ = 21511990.

The previous prime is 21511979. The next prime is 21511993. The reversal of 21511991 is 19911512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21511991 - 2⁶ = 21511927 is a prime.

Together with 21511993, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21511993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10755995 + 10755996.

It is an arithmetic number, because the mean of its divisors is an integer number (10755996).

Almost surely, 2^21511991 is an apocalyptic number.

21511991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21511991 is an equidigital number, since it uses as much as digits as its factorization.

21511991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 810, while the sum is 29.

The square root of 21511991 is about 4638.1020902951. The cubic root of 21511991 is about 278.1165729215.

The spelling of 21511991 in words is "twenty-one million, five hundred eleven thousand, nine hundred ninety-one".