18520721 has 4 divisors (see below), whose sum is σ = 18547200. Its totient is φ = 18494244.

The previous prime is 18520717. The next prime is 18520729. The reversal of 18520721 is 12702581.

18520721 is nontrivially palindromic in base 4.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12702581 = 37 ⋅343313.

It is a cyclic number.

It is not a de Polignac number, because 18520721 - 2² = 18520717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (18520729) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 12161 + ... + 13598.

It is an arithmetic number, because the mean of its divisors is an integer number (4636800).

Almost surely, 2^18520721 is an apocalyptic number.

It is an amenable number.

18520721 is a deficient number, since it is larger than the sum of its proper divisors (26479).

18520721 is an equidigital number, since it uses as much as digits as its factorization.

18520721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 26478.

The product of its (nonzero) digits is 1120, while the sum is 26.

The square root of 18520721 is about 4303.5707267338. The cubic root of 18520721 is about 264.5773302432.

The spelling of 18520721 in words is "eighteen million, five hundred twenty thousand, seven hundred twenty-one".