18156721 has 4 divisors (see below), whose sum is σ = 19807344. Its totient is φ = 16506100.

The previous prime is 18156709. The next prime is 18156739. The reversal of 18156721 is 12765181.

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 a cyclic number.

It is not a de Polignac number, because 18156721 - 2⁹ = 18156209 is a prime.

It is a super-3 number, since 3×18156721³ (a number of 23 digits) contains 333 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18156701) 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, 825295 + ... + 825316.

It is an arithmetic number, because the mean of its divisors is an integer number (4951836).

Almost surely, 2^18156721 is an apocalyptic number.

18156721 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

18156721 is a deficient number, since it is larger than the sum of its proper divisors (1650623).

18156721 is a wasteful number, since it uses less digits than its factorization.

18156721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1650622.

The product of its digits is 3360, while the sum is 31.

The square root of 18156721 is about 4261.0704054263. The cubic root of 18156721 is about 262.8325448306.

The spelling of 18156721 in words is "eighteen million, one hundred fifty-six thousand, seven hundred twenty-one".