1638391 has 4 divisors (see below), whose sum is σ = 1640952. Its totient is φ = 1635832.

The previous prime is 1638353. The next prime is 1638431. The reversal of 1638391 is 1938361.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 1938361 = 19 ⋅102019.

It is a cyclic number.

It is not a de Polignac number, because 1638391 - 2¹¹ = 1636343 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1638311) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 636 + ... + 1918.

It is an arithmetic number, because the mean of its divisors is an integer number (410238).

2^1638391 is an apocalyptic number.

1638391 is a deficient number, since it is larger than the sum of its proper divisors (2561).

1638391 is a wasteful number, since it uses less digits than its factorization.

1638391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2560.

The product of its digits is 3888, while the sum is 31.

The square root of 1638391 is about 1279.9964843702. The cubic root of 1638391 is about 117.8887920949. Note that the first 3 decimals are identical.

The spelling of 1638391 in words is "one million, six hundred thirty-eight thousand, three hundred ninety-one".