1638721 has 4 divisors (see below), whose sum is σ = 1872832. Its totient is φ = 1404612.

The previous prime is 1638719. The next prime is 1638733. The reversal of 1638721 is 1278361.

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 1638721 - 2¹ = 1638719 is a prime.

It is an Ulam number.

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

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 1638721.

It is not an unprimeable number, because it can be changed into a prime (1638701) 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, 117045 + ... + 117058.

It is an arithmetic number, because the mean of its divisors is an integer number (468208).

2^1638721 is an apocalyptic number.

It is an amenable number.

1638721 is a deficient number, since it is larger than the sum of its proper divisors (234111).

1638721 is an equidigital number, since it uses as much as digits as its factorization.

1638721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 234110.

The product of its digits is 2016, while the sum is 28.

The square root of 1638721 is about 1280.1253844839. The cubic root of 1638721 is about 117.8967065039.

The spelling of 1638721 in words is "one million, six hundred thirty-eight thousand, seven hundred twenty-one".