21817991 has 8 divisors (see below), whose sum is σ = 23548560. Its totient is φ = 20094912.

The previous prime is 21817981. The next prime is 21818003. The reversal of 21817991 is 19971812.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21817991 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 (21817951) 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, 5195 + ... + 8403.

It is an arithmetic number, because the mean of its divisors is an integer number (2943570).

Almost surely, 2^21817991 is an apocalyptic number.

21817991 is a deficient number, since it is larger than the sum of its proper divisors (1730569).

21817991 is a wasteful number, since it uses less digits than its factorization.

21817991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3745.

The product of its digits is 9072, while the sum is 38.

The square root of 21817991 is about 4670.9732390584. The cubic root of 21817991 is about 279.4290704901.

The spelling of 21817991 in words is "twenty-one million, eight hundred seventeen thousand, nine hundred ninety-one".