21010991 has 4 divisors (see below), whose sum is σ = 21020184. Its totient is φ = 21001800.

The previous prime is 21010987. The next prime is 21011009. The reversal of 21010991 is 19901012.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21010991 - 2² = 21010987 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21010921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1796 + ... + 6726.

It is an arithmetic number, because the mean of its divisors is an integer number (5255046).

Almost surely, 2^21010991 is an apocalyptic number.

21010991 is a deficient number, since it is larger than the sum of its proper divisors (9193).

21010991 is an equidigital number, since it uses as much as digits as its factorization.

21010991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9192.

The product of its (nonzero) digits is 162, while the sum is 23.

The square root of 21010991 is about 4583.7747545009. The cubic root of 21010991 is about 275.9405415221.

The spelling of 21010991 in words is "twenty-one million, ten thousand, nine hundred ninety-one".