21718 has 4 divisors (see below), whose sum is σ = 32580. Its totient is φ = 10858.

The previous prime is 21713. The next prime is 21727. The reversal of 21718 is 81712.

It is a semiprime because it is the product of two primes.

It is a plaindrome in base 15.

It is a junction number, because it is equal to n+sod(n) for n = 21695 and 21704.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21713) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5428 + ... + 5431.

It is an arithmetic number, because the mean of its divisors is an integer number (8145).

2²¹⁷¹⁸ is an apocalyptic number.

21718 is a deficient number, since it is larger than the sum of its proper divisors (10862).

21718 is a wasteful number, since it uses less digits than its factorization.

21718 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10861.

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

The square root of 21718 is about 147.3702819431. The cubic root of 21718 is about 27.9001545878.

The spelling of 21718 in words is "twenty-one thousand, seven hundred eighteen".