21791 has 8 divisors (see below), whose sum is σ = 27264. Its totient is φ = 16920.

The previous prime is 21787. The next prime is 21799. The reversal of 21791 is 19712.

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 21791 - 2² = 21787 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21799) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 65 + ... + 218.

It is an arithmetic number, because the mean of its divisors is an integer number (3408).

2²¹⁷⁹¹ is an apocalyptic number.

21791 is a deficient number, since it is larger than the sum of its proper divisors (5473).

21791 is a wasteful number, since it uses less digits than its factorization.

21791 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 301.

The product of its digits is 126, while the sum is 20.

The square root of 21791 is about 147.6177496103. The cubic root of 21791 is about 27.9313795864.

The spelling of 21791 in words is "twenty-one thousand, seven hundred ninety-one".