21001999 has 4 divisors (see below), whose sum is σ = 21012040. Its totient is φ = 20991960.

The previous prime is 21001991. The next prime is 21002039. The reversal of 21001999 is 99910012.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21001999 - 2³ = 21001991 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21001999.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001991) 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, 564 + ... + 6505.

It is an arithmetic number, because the mean of its divisors is an integer number (5253010).

Almost surely, 2^21001999 is an apocalyptic number.

21001999 is a deficient number, since it is larger than the sum of its proper divisors (10041).

21001999 is an equidigital number, since it uses as much as digits as its factorization.

21001999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10040.

The product of its (nonzero) digits is 1458, while the sum is 31.

The square root of 21001999 is about 4582.7937985469. The cubic root of 21001999 is about 275.9011714706.

It can be divided in two parts, 2 and 1001999, that added together give a palindrome (1002001).

The spelling of 21001999 in words is "twenty-one million, one thousand, nine hundred ninety-nine".