21089991 has 8 divisors (see below), whose sum is σ = 30283120. Its totient is φ = 12978432.

The previous prime is 21089987. The next prime is 21089993. The reversal of 21089991 is 19998012.

It is a happy number.

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

It is not a de Polignac number, because 21089991 - 2² = 21089987 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (39), and also a Moran number because the ratio is a prime number: 540769 = 21089991 / (2 + 1 + 0 + 8 + 9 + 9 + 9 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21089993) 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 7 ways as a sum of consecutive naturals, for example, 270346 + ... + 270423.

It is an arithmetic number, because the mean of its divisors is an integer number (3785390).

Almost surely, 2^21089991 is an apocalyptic number.

21089991 is a deficient number, since it is larger than the sum of its proper divisors (9193129).

21089991 is a wasteful number, since it uses less digits than its factorization.

21089991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 540785.

The product of its (nonzero) digits is 11664, while the sum is 39.

The square root of 21089991 is about 4592.3840213989. The cubic root of 21089991 is about 276.2859486533.

The spelling of 21089991 in words is "twenty-one million, eighty-nine thousand, nine hundred ninety-one".