21821991 has 4 divisors (see below), whose sum is σ = 29095992. Its totient is φ = 14547992.

The previous prime is 21821981. The next prime is 21822001. The reversal of 21821991 is 19912812.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21821981) and next prime (21822001).

It is a cyclic number.

It is not a de Polignac number, because 21821991 - 2⁶ = 21821927 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21821911) 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, 3636996 + ... + 3637001.

It is an arithmetic number, because the mean of its divisors is an integer number (7273998).

Almost surely, 2^21821991 is an apocalyptic number.

21821991 is a deficient number, since it is larger than the sum of its proper divisors (7274001).

21821991 is an equidigital number, since it uses as much as digits as its factorization.

21821991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7274000.

The product of its digits is 2592, while the sum is 33.

The square root of 21821991 is about 4671.4013957270. The cubic root of 21821991 is about 279.4461458170.

The spelling of 21821991 in words is "twenty-one million, eight hundred twenty-one thousand, nine hundred ninety-one".