18217721 has 4 divisors (see below), whose sum is σ = 18226512. Its totient is φ = 18208932.

The previous prime is 18217709. The next prime is 18217723. The reversal of 18217721 is 12771281.

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 a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 12771281 = 29 ⋅440389.

It is a cyclic number.

It is not a de Polignac number, because 18217721 - 2⁶ = 18217657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18217723) 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, 626 + ... + 6068.

It is an arithmetic number, because the mean of its divisors is an integer number (4556628).

Almost surely, 2^18217721 is an apocalyptic number.

It is an amenable number.

18217721 is a deficient number, since it is larger than the sum of its proper divisors (8791).

18217721 is an equidigital number, since it uses as much as digits as its factorization.

18217721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8790.

The product of its digits is 1568, while the sum is 29.

The square root of 18217721 is about 4268.2222294534. Note that the first 5 decimals coincide. The cubic root of 18217721 is about 263.1265565061.

The spelling of 18217721 in words is "eighteen million, two hundred seventeen thousand, seven hundred twenty-one".