21781 has 4 divisors (see below), whose sum is σ = 22752. Its totient is φ = 20812.

The previous prime is 21773. The next prime is 21787. The reversal of 21781 is 18712.

21781 = T₁₁₉ + T₁₂₀ + T₁₂₁.

21781 is nontrivially palindromic in base 8.

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 21781 - 2³ = 21773 is a prime.

It is a super-2 number, since 2×21781² = 948823922, which contains 22 as substring.

It is a Duffinian number.

It is a zygodrome in base 5.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21787) 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 3 ways as a sum of consecutive naturals, for example, 451 + ... + 496.

It is an arithmetic number, because the mean of its divisors is an integer number (5688).

2²¹⁷⁸¹ is an apocalyptic number.

21781 is the 121-st centered triangular number.

It is an amenable number.

21781 is a deficient number, since it is larger than the sum of its proper divisors (971).

21781 is an equidigital number, since it uses as much as digits as its factorization.

21781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 970.

The product of its digits is 112, while the sum is 19.

The square root of 21781 is about 147.5838744579. The cubic root of 21781 is about 27.9271063156.

The spelling of 21781 in words is "twenty-one thousand, seven hundred eighty-one".