18999721 has 4 divisors (see below), whose sum is σ = 20461252. Its totient is φ = 17538192.

The previous prime is 18999709. The next prime is 18999763. The reversal of 18999721 is 12799981.

It is a happy number.

It is the 1780-th star number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 6574096 + 12425625 = 2564^2 + 3525^2 .

It is a cyclic number.

It is not a de Polignac number, because 18999721 - 2⁹ = 18999209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18999821) 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, 730746 + ... + 730771.

It is an arithmetic number, because the mean of its divisors is an integer number (5115313).

Almost surely, 2^18999721 is an apocalyptic number.

It is an amenable number.

18999721 is a deficient number, since it is larger than the sum of its proper divisors (1461531).

18999721 is a wasteful number, since it uses less digits than its factorization.

18999721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1461530.

The product of its digits is 81648, while the sum is 46.

The square root of 18999721 is about 4358.8669399283. The cubic root of 18999721 is about 266.8388587534.

The spelling of 18999721 in words is "eighteen million, nine hundred ninety-nine thousand, seven hundred twenty-one".