21451 has 4 divisors (see below), whose sum is σ = 22600. Its totient is φ = 20304.

The previous prime is 21433. The next prime is 21467. The reversal of 21451 is 15412.

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 21451 - 2⁵ = 21419 is a prime.

It is a Duffinian number.

21451 is a lucky number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21401) 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, 546 + ... + 583.

It is an arithmetic number, because the mean of its divisors is an integer number (5650).

2²¹⁴⁵¹ is an apocalyptic number.

21451 is the 66-th centered decagonal number.

21451 is a deficient number, since it is larger than the sum of its proper divisors (1149).

21451 is a wasteful number, since it uses less digits than its factorization.

21451 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1148.

The product of its digits is 40, while the sum is 13.

The square root of 21451 is about 146.4615990627. The cubic root of 21451 is about 27.7853484550.

Adding to 21451 its reverse (15412), we get a palindrome (36863).

The spelling of 21451 in words is "twenty-one thousand, four hundred fifty-one".