21091 has 8 divisors (see below), whose sum is σ = 25344. Its totient is φ = 17160.

The previous prime is 21089. The next prime is 21101. The reversal of 21091 is 19012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21091 - 2¹ = 21089 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21001) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 96 + ... + 226.

It is an arithmetic number, because the mean of its divisors is an integer number (3168).

2²¹⁰⁹¹ is an apocalyptic number.

21091 is a deficient number, since it is larger than the sum of its proper divisors (4253).

21091 is a wasteful number, since it uses less digits than its factorization.

21091 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 161.

The product of its (nonzero) digits is 18, while the sum is 13.

The square root of 21091 is about 145.2274078816. The cubic root of 21091 is about 27.6290354662.

It can be divided in two parts, 210 and 91, that multiplied together give a triangular number (19110 = T₁₉₅).

The spelling of 21091 in words is "twenty-one thousand, ninety-one".