21561999 has 4 divisors (see below), whose sum is σ = 28749336. Its totient is φ = 14374664.

The previous prime is 21561997. The next prime is 21562003. The reversal of 21561999 is 99916512.

It is a happy number.

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

It is a cyclic number.

It is not a de Polignac number, because 21561999 - 2¹ = 21561997 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21561999.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21561997) 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, 3593664 + ... + 3593669.

It is an arithmetic number, because the mean of its divisors is an integer number (7187334).

Almost surely, 2^21561999 is an apocalyptic number.

21561999 is a deficient number, since it is larger than the sum of its proper divisors (7187337).

21561999 is an equidigital number, since it uses as much as digits as its factorization.

21561999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7187336.

The product of its digits is 43740, while the sum is 42.

The square root of 21561999 is about 4643.4899590717. The cubic root of 21561999 is about 278.3319146863.

The spelling of 21561999 in words is "twenty-one million, five hundred sixty-one thousand, nine hundred ninety-nine".