21691091 has 4 divisors (see below), whose sum is σ = 22220184. Its totient is φ = 21162000.

The previous prime is 21691081. The next prime is 21691097. The reversal of 21691091 is 19019612.

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 21691091 - 2¹⁸ = 21428947 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21691097) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 264485 + ... + 264566.

It is an arithmetic number, because the mean of its divisors is an integer number (5555046).

Almost surely, 2^21691091 is an apocalyptic number.

21691091 is a deficient number, since it is larger than the sum of its proper divisors (529093).

21691091 is an equidigital number, since it uses as much as digits as its factorization.

21691091 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 529092.

The product of its (nonzero) digits is 972, while the sum is 29.

The square root of 21691091 is about 4657.3695365517. The cubic root of 21691091 is about 278.8862689117.

The spelling of 21691091 in words is "twenty-one million, six hundred ninety-one thousand, ninety-one".