21101891 has 4 divisors (see below), whose sum is σ = 21391032. Its totient is φ = 20812752.

The previous prime is 21101887. The next prime is 21101903. The reversal of 21101891 is 19810112.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21101891 - 2² = 21101887 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21181891) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 144461 + ... + 144606.

It is an arithmetic number, because the mean of its divisors is an integer number (5347758).

Almost surely, 2^21101891 is an apocalyptic number.

21101891 is a deficient number, since it is larger than the sum of its proper divisors (289141).

21101891 is an equidigital number, since it uses as much as digits as its factorization.

21101891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 289140.

The product of its (nonzero) digits is 144, while the sum is 23.

The square root of 21101891 is about 4593.6794620435. The cubic root of 21101891 is about 276.3379035452.

The spelling of 21101891 in words is "twenty-one million, one hundred one thousand, eight hundred ninety-one".