21009991 has 8 divisors (see below), whose sum is σ = 22292480. Its totient is φ = 19745208.

The previous prime is 21009983. The next prime is 21009997. The reversal of 21009991 is 19990012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21009991 - 2³ = 21009983 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 (21009997) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1941 + ... + 6766.

It is an arithmetic number, because the mean of its divisors is an integer number (2786560).

Almost surely, 2^21009991 is an apocalyptic number.

21009991 is a deficient number, since it is larger than the sum of its proper divisors (1282489).

21009991 is a wasteful number, since it uses less digits than its factorization.

21009991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8853.

The product of its (nonzero) digits is 1458, while the sum is 31.

The square root of 21009991 is about 4583.6656727994. The cubic root of 21009991 is about 275.9361637353.

The spelling of 21009991 in words is "twenty-one million, nine thousand, nine hundred ninety-one".