21781891 has 2 divisors, whose sum is σ = 21781892. Its totient is φ = 21781890.

The previous prime is 21781873. The next prime is 21781967. The reversal of 21781891 is 19818712.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21781891 - 2⁹ = 21781379 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21781891.

It is not a weakly prime, because it can be changed into another prime (21784891) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10890945 + 10890946.

It is an arithmetic number, because the mean of its divisors is an integer number (10890946).

Almost surely, 2^21781891 is an apocalyptic number.

21781891 is a deficient number, since it is larger than the sum of its proper divisors (1).

21781891 is an equidigital number, since it uses as much as digits as its factorization.

21781891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8064, while the sum is 37.

The square root of 21781891 is about 4667.1073482404. The cubic root of 21781891 is about 279.2748711707.

The spelling of 21781891 in words is "twenty-one million, seven hundred eighty-one thousand, eight hundred ninety-one".