21251 has 4 divisors (see below), whose sum is σ = 21600. Its totient is φ = 20904.

The previous prime is 21247. The next prime is 21269. The reversal of 21251 is 15212.

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 21251 - 2² = 21247 is a prime.

It is a Duffinian number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21251.

It is not an unprimeable number, because it can be changed into a prime (21211) 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, 56 + ... + 213.

It is an arithmetic number, because the mean of its divisors is an integer number (5400).

2²¹²⁵¹ is an apocalyptic number.

21251 is a deficient number, since it is larger than the sum of its proper divisors (349).

21251 is an equidigital number, since it uses as much as digits as its factorization.

21251 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 348.

The product of its digits is 20, while the sum is 11.

The square root of 21251 is about 145.7772273025. Note that the first 3 decimals coincide. The cubic root of 21251 is about 27.6987257600.

Adding to 21251 its reverse (15212), we get a palindrome (36463).

It can be divided in two parts, 21 and 251, that added together give a palindrome (272).

The spelling of 21251 in words is "twenty-one thousand, two hundred fifty-one".