21991559 has 4 divisors (see below), whose sum is σ = 22012704. Its totient is φ = 21970416.

The previous prime is 21991517. The next prime is 21991561. The reversal of 21991559 is 95519912.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21991559 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 (21991589) 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, 8927 + ... + 11120.

It is an arithmetic number, because the mean of its divisors is an integer number (5503176).

Almost surely, 2^21991559 is an apocalyptic number.

21991559 is a deficient number, since it is larger than the sum of its proper divisors (21145).

21991559 is a wasteful number, since it uses less digits than its factorization.

21991559 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 21144.

The product of its digits is 36450, while the sum is 41.

The square root of 21991559 is about 4689.5158598730. The cubic root of 21991559 is about 280.1680920965.

The spelling of 21991559 in words is "twenty-one million, nine hundred ninety-one thousand, five hundred fifty-nine".