21780991 has 2 divisors, whose sum is σ = 21780992. Its totient is φ = 21780990.

The previous prime is 21780989. The next prime is 21781021. The reversal of 21780991 is 19908712.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21780991 - 2¹ = 21780989 is a prime.

Together with 21780989, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21780991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21780491) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10890495 + 10890496.

It is an arithmetic number, because the mean of its divisors is an integer number (10890496).

Almost surely, 2^21780991 is an apocalyptic number.

21780991 is the 2695-th hex number.

21780991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21780991 is an equidigital number, since it uses as much as digits as its factorization.

21780991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 9072, while the sum is 37.

The square root of 21780991 is about 4667.0109277781. The cubic root of 21780991 is about 279.2710246904.

The spelling of 21780991 in words is "twenty-one million, seven hundred eighty thousand, nine hundred ninety-one".