18021661 has 6 divisors (see below), whose sum is σ = 20964030. Its totient is φ = 15447096.

The previous prime is 18021637. The next prime is 18021667. The reversal of 18021661 is 16612081.

It can be written as a sum of positive squares in only one way, i.e., 17935225 + 86436 = 4235^2 + 294^2 .

It is not a de Polignac number, because 18021661 - 2⁵ = 18021629 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18021667) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 183846 + ... + 183943.

It is an arithmetic number, because the mean of its divisors is an integer number (3494005).

Almost surely, 2^18021661 is an apocalyptic number.

It is an amenable number.

18021661 is a deficient number, since it is larger than the sum of its proper divisors (2942369).

18021661 is an equidigital number, since it uses as much as digits as its factorization.

18021661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 367803 (or 367796 counting only the distinct ones).

The product of its (nonzero) digits is 576, while the sum is 25.

The square root of 18021661 is about 4245.1926929175. The cubic root of 18021661 is about 262.1792229825.

The spelling of 18021661 in words is "eighteen million, twenty-one thousand, six hundred sixty-one".