21517 has 2 divisors, whose sum is σ = 21518. Its totient is φ = 21516.

The previous prime is 21503. The next prime is 21521. The reversal of 21517 is 71512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17161 + 4356 = 131^2 + 66^2 .

It is a cyclic number.

It is not a de Polignac number, because 21517 - 2¹¹ = 19469 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21517.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21557) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10758 + 10759.

It is an arithmetic number, because the mean of its divisors is an integer number (10759).

2²¹⁵¹⁷ is an apocalyptic number.

It is an amenable number.

21517 is a deficient number, since it is larger than the sum of its proper divisors (1).

21517 is an equidigital number, since it uses as much as digits as its factorization.

21517 is an evil number, because the sum of its binary digits is even.

The product of its digits is 70, while the sum is 16.

The square root of 21517 is about 146.6867410504. The cubic root of 21517 is about 27.8138157438.

It can be divided in two parts, 215 and 17, that multiplied together give a triangular number (3655 = T₈₅).

The spelling of 21517 in words is "twenty-one thousand, five hundred seventeen".