21518 has 16 divisors (see below), whose sum is σ = 38880. Its totient is φ = 8736.

The previous prime is 21517. The next prime is 21521. The reversal of 21518 is 81512.

21518 is nontrivially palindromic in base 6.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21496 and 21505.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21517) 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, 380 + ... + 432.

It is an arithmetic number, because the mean of its divisors is an integer number (2430).

2²¹⁵¹⁸ is an apocalyptic number.

21518 is a deficient number, since it is larger than the sum of its proper divisors (17362).

21518 is a wasteful number, since it uses less digits than its factorization.

21518 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 91.

The product of its digits is 80, while the sum is 17.

The square root of 21518 is about 146.6901496352. The cubic root of 21518 is about 27.8142466184.

Multiplying 21518 by its product of digits (80), we get a triangular number (1721440 = T₁₈₅₅).

It can be divided in two parts, 21 and 518, that multiplied together give a triangular number (10878 = T₁₄₇).

The spelling of 21518 in words is "twenty-one thousand, five hundred eighteen".