21421 has 4 divisors (see below), whose sum is σ = 22144. Its totient is φ = 20700.

The previous prime is 21419. The next prime is 21433. The reversal of 21421 is 12412.

21421 = T₁₁₈ + T₁₁₉ + T₁₂₀.

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 21421 - 2¹ = 21419 is a prime.

It is a Duffinian number.

21421 is a lucky number.

It is a plaindrome in base 13.

It is a nialpdrome in base 15.

It is a junction number, because it is equal to n+sod(n) for n = 21398 and 21407.

It is a congruent number.

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, 315 + ... + 376.

It is an arithmetic number, because the mean of its divisors is an integer number (5536).

2²¹⁴²¹ is an apocalyptic number.

21421 is the 120-th centered triangular number and also the 85-th hex number.

It is an amenable number.

21421 is a deficient number, since it is larger than the sum of its proper divisors (723).

21421 is an equidigital number, since it uses as much as digits as its factorization.

21421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 722.

The product of its digits is 16, while the sum is 10.

The square root of 21421 is about 146.3591473055. The cubic root of 21421 is about 27.7723894734.

Adding to 21421 its reverse (12412), we get a palindrome (33833).

Subtracting from 21421 its reverse (12412), we obtain a palindrome (9009).

The spelling of 21421 in words is "twenty-one thousand, four hundred twenty-one", and thus it is an iban number.