21553 has 4 divisors (see below), whose sum is σ = 24640. Its totient is φ = 18468.

The previous prime is 21529. The next prime is 21557. The reversal of 21553 is 35512.

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 21553 - 2⁵ = 21521 is a prime.

It is a nialpdrome in base 16.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21557) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1533 + ... + 1546.

It is an arithmetic number, because the mean of its divisors is an integer number (6160).

2²¹⁵⁵³ is an apocalyptic number.

It is an amenable number.

21553 is a deficient number, since it is larger than the sum of its proper divisors (3087).

21553 is an equidigital number, since it uses as much as digits as its factorization.

21553 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3086.

The product of its digits is 150, while the sum is 16.

The square root of 21553 is about 146.8094002440. The cubic root of 21553 is about 27.8293188261.

The spelling of 21553 in words is "twenty-one thousand, five hundred fifty-three".