21105891 has 12 divisors (see below), whose sum is σ = 32279832. Its totient is φ = 13242816.

The previous prime is 21105863. The next prime is 21105907. The reversal of 21105891 is 19850112.

It is not a de Polignac number, because 21105891 - 2¹¹ = 21103843 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21105811) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 68821 + ... + 69126.

It is an arithmetic number, because the mean of its divisors is an integer number (2689986).

Almost surely, 2^21105891 is an apocalyptic number.

21105891 is a deficient number, since it is larger than the sum of its proper divisors (11173941).

21105891 is a wasteful number, since it uses less digits than its factorization.

21105891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 137970 (or 137967 counting only the distinct ones).

The product of its (nonzero) digits is 720, while the sum is 27.

The square root of 21105891 is about 4594.1148222481. The cubic root of 21105891 is about 276.3553629880.

It can be divided in two parts, 2110 and 5891, that added together give a triangular number (8001 = T₁₂₆).

The spelling of 21105891 in words is "twenty-one million, one hundred five thousand, eight hundred ninety-one".