18721661 has 4 divisors (see below), whose sum is σ = 21396192. Its totient is φ = 16047132.

The previous prime is 18721649. The next prime is 18721673. The reversal of 18721661 is 16612781.

It is a happy number.

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.

It is an interprime number because it is at equal distance from previous prime (18721649) and next prime (18721673).

It is a cyclic number.

It is not a de Polignac number, because 18721661 - 2¹⁴ = 18705277 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18721601) 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, 1337255 + ... + 1337268.

It is an arithmetic number, because the mean of its divisors is an integer number (5349048).

Almost surely, 2^18721661 is an apocalyptic number.

It is an amenable number.

18721661 is a deficient number, since it is larger than the sum of its proper divisors (2674531).

18721661 is an equidigital number, since it uses as much as digits as its factorization.

18721661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2674530.

The product of its digits is 4032, while the sum is 32.

The square root of 18721661 is about 4326.8534756795. The cubic root of 18721661 is about 265.5307317807.

The spelling of 18721661 in words is "eighteen million, seven hundred twenty-one thousand, six hundred sixty-one".