165621 has 4 divisors (see below), whose sum is σ = 220832. Its totient is φ = 110412.

The previous prime is 165617. The next prime is 165653. The reversal of 165621 is 126561.

It is a happy number.

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, and also an emirpimes, since its reverse is a distinct semiprime: 126561 = 3 ⋅42187.

It is not a de Polignac number, because 165621 - 2² = 165617 is a prime.

It is a D-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 (165601) 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, 27601 + ... + 27606.

It is an arithmetic number, because the mean of its divisors is an integer number (55208).

2¹⁶⁵⁶²¹ is an apocalyptic number.

It is an amenable number.

165621 is a deficient number, since it is larger than the sum of its proper divisors (55211).

165621 is an equidigital number, since it uses as much as digits as its factorization.

165621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 55210.

The product of its digits is 360, while the sum is 21.

The square root of 165621 is about 406.9656005119. The cubic root of 165621 is about 54.9167887712.

Subtracting from 165621 its sum of digits (21), we obtain a triangular number (165600 = T₅₇₅).

Subtracting from 165621 its reverse (126561), we obtain a triangular number (39060 = T₂₇₉).

The spelling of 165621 in words is "one hundred sixty-five thousand, six hundred twenty-one".