21931 has 8 divisors (see below), whose sum is σ = 27104. Its totient is φ = 17280.

The previous prime is 21929. The next prime is 21937. The reversal of 21931 is 13912.

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

It is a 7-Lehmer number, since φ(21931) divides (21931-1)⁷.

It is a cyclic number.

It is not a de Polignac number, because 21931 - 2¹ = 21929 is a prime.

It is a super-2 number, since 2×21931² = 961937522, which contains 22 as substring.

It is a deceptive number, since it divides R₂₁₉₃₀.

21931 is a lucky number.

It is a plaindrome in base 4 and base 16.

It is not an unprimeable number, because it can be changed into a prime (21937) 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, 30 + ... + 211.

It is an arithmetic number, because the mean of its divisors is an integer number (3388).

2²¹⁹³¹ is an apocalyptic number.

21931 is the 86-th hex number.

21931 is a deficient number, since it is larger than the sum of its proper divisors (5173).

21931 is a wasteful number, since it uses less digits than its factorization.

21931 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 261.

The product of its digits is 54, while the sum is 16.

The square root of 21931 is about 148.0911881241. The cubic root of 21931 is about 27.9910685799.

The spelling of 21931 in words is "twenty-one thousand, nine hundred thirty-one".