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18721 = 97193

18721 has 4 divisors (see below), whose sum is σ = 19012. Its totient is φ = 18432.

The previous prime is 18719. The next prime is 18731. The reversal of 18721 is 12781.

18721 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

18721 is a nontrivial binomial coefficient, being equal to C(194, 2).

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 6400 + 12321 = 80^2 + 111^2 .

It is a 3-Lehmer number, since φ(18721) divides (18721-1)3.

It is a cyclic number.

It is not a de Polignac number, because 18721 - 21 = 18719 is a prime.

It is an alternating number because its digits alternate between odd and even.

It is a nialpdrome in base 8.

It is not an unprimeable number, because it can be changed into a prime (18701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 193.

It is an arithmetic number, because the mean of its divisors is an integer number (4753).

It is a Poulet number, since it divides 218720-1.

218721 is an apocalyptic number.

18721 is the 193-rd triangular number and also the 97-th hexagonal number.

18721 is the 65-th centered nonagonal number.

It is an amenable number.

18721 is a deficient number, since it is larger than the sum of its proper divisors (291).

18721 is an equidigital number, since it uses as much as digits as its factorization.

18721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 290.

The product of its digits is 112, while the sum is 19.

The square root of 18721 is about 136.8247053715. The cubic root of 18721 is about 26.5527606740.

The spelling of 18721 in words is "eighteen thousand, seven hundred twenty-one".

Divisors: 1 97 193 18721