21181891 has 2 divisors, whose sum is σ = 21181892. Its totient is φ = 21181890.

The previous prime is 21181889. The next prime is 21181907. The reversal of 21181891 is 19818112.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21181891 - 2¹ = 21181889 is a prime.

Together with 21181889, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21181801) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10590945 + 10590946.

It is an arithmetic number, because the mean of its divisors is an integer number (10590946).

Almost surely, 2^21181891 is an apocalyptic number.

21181891 is a deficient number, since it is larger than the sum of its proper divisors (1).

21181891 is an equidigital number, since it uses as much as digits as its factorization.

21181891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1152, while the sum is 31.

The square root of 21181891 is about 4602.3788414254. The cubic root of 21181891 is about 276.6866740906.

The spelling of 21181891 in words is "twenty-one million, one hundred eighty-one thousand, eight hundred ninety-one".