165905821 has 4 divisors (see below), whose sum is σ = 165946144. Its totient is φ = 165865500.

The previous prime is 165905819. The next prime is 165905827. The reversal of 165905821 is 128509561.

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 165905821 - 2¹ = 165905819 is a prime.

It is a super-3 number, since 3×165905821³ (a number of 26 digits) contains 333 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (165905827) 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, 13185 + ... + 22486.

It is an arithmetic number, because the mean of its divisors is an integer number (41486536).

Almost surely, 2^165905821 is an apocalyptic number.

It is an amenable number.

165905821 is a deficient number, since it is larger than the sum of its proper divisors (40323).

165905821 is an equidigital number, since it uses as much as digits as its factorization.

165905821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 40322.

The product of its (nonzero) digits is 21600, while the sum is 37.

The square root of 165905821 is about 12880.4433541707. The cubic root of 165905821 is about 549.4825116012.

The spelling of 165905821 in words is "one hundred sixty-five million, nine hundred five thousand, eight hundred twenty-one".