1638160721 has 4 divisors (see below), whose sum is σ = 1640923812. Its totient is φ = 1635397632.

The previous prime is 1638160697. The next prime is 1638160739. The reversal of 1638160721 is 1270618361.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1270618361 = 1741 ⋅729821.

It can be written as a sum of positive squares in 2 ways, for example, as 1259469121 + 378691600 = 35489^2 + 19460^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1638160721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1638160751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1380656 + ... + 1381841.

It is an arithmetic number, because the mean of its divisors is an integer number (410230953).

Almost surely, 2^1638160721 is an apocalyptic number.

It is an amenable number.

1638160721 is a deficient number, since it is larger than the sum of its proper divisors (2763091).

1638160721 is an equidigital number, since it uses as much as digits as its factorization.

1638160721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2763090.

The product of its (nonzero) digits is 12096, while the sum is 35.

The square root of 1638160721 is about 40474.1982131827. The cubic root of 1638160721 is about 1178.8326867075.

The spelling of 1638160721 in words is "one billion, six hundred thirty-eight million, one hundred sixty thousand, seven hundred twenty-one".