21906691 has 4 divisors (see below), whose sum is σ = 21937552. Its totient is φ = 21875832.

The previous prime is 21906673. The next prime is 21906697. The reversal of 21906691 is 19660912.

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 21906691 - 2⁹ = 21906179 is a prime.

It is a super-2 number, since 2×21906691² = 959806221138962, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21906697) 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, 14340 + ... + 15793.

It is an arithmetic number, because the mean of its divisors is an integer number (5484388).

Almost surely, 2^21906691 is an apocalyptic number.

21906691 is a deficient number, since it is larger than the sum of its proper divisors (30861).

21906691 is an equidigital number, since it uses as much as digits as its factorization.

21906691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 30860.

The product of its (nonzero) digits is 5832, while the sum is 34.

The square root of 21906691 is about 4680.4584177194. The cubic root of 21906691 is about 279.8072270050.

The spelling of 21906691 in words is "twenty-one million, nine hundred six thousand, six hundred ninety-one".