21551 has 4 divisors (see below), whose sum is σ = 22512. Its totient is φ = 20592.

The previous prime is 21529. The next prime is 21557. The reversal of 21551 is 15512.

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

It is a cyclic number.

It is not a de Polignac number, because 21551 - 2⁶ = 21487 is a prime.

It is a Duffinian number.

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, 446 + ... + 491.

It is an arithmetic number, because the mean of its divisors is an integer number (5628).

2²¹⁵⁵¹ is an apocalyptic number.

21551 is a deficient number, since it is larger than the sum of its proper divisors (961).

21551 is an equidigital number, since it uses as much as digits as its factorization.

21551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 960.

The product of its digits is 50, while the sum is 14.

The square root of 21551 is about 146.8025885330. The cubic root of 21551 is about 27.8284579968.

The spelling of 21551 in words is "twenty-one thousand, five hundred fifty-one".