21817 has 2 divisors, whose sum is σ = 21818. Its totient is φ = 21816.

The previous prime is 21803. The next prime is 21821. The reversal of 21817 is 71812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19881 + 1936 = 141^2 + 44^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21817 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21794 and 21803.

It is not a weakly prime, because it can be changed into another prime (21017) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10908 + 10909.

It is an arithmetic number, because the mean of its divisors is an integer number (10909).

2²¹⁸¹⁷ is an apocalyptic number.

It is an amenable number.

21817 is a deficient number, since it is larger than the sum of its proper divisors (1).

21817 is an equidigital number, since it uses as much as digits as its factorization.

21817 is an evil number, because the sum of its binary digits is even.

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

The square root of 21817 is about 147.7057886476. The cubic root of 21817 is about 27.9424839755.

It can be divided in two parts, 21 and 817, that added together give a palindrome (838).

The spelling of 21817 in words is "twenty-one thousand, eight hundred seventeen".