21216421 has 4 divisors (see below), whose sum is σ = 21533152. Its totient is φ = 20899692.

The previous prime is 21216413. The next prime is 21216427. The reversal of 21216421 is 12461212.

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 21216421 - 2³ = 21216413 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 (21216427) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 158265 + ... + 158398.

It is an arithmetic number, because the mean of its divisors is an integer number (5383288).

Almost surely, 2^21216421 is an apocalyptic number.

It is an amenable number.

21216421 is a deficient number, since it is larger than the sum of its proper divisors (316731).

21216421 is an equidigital number, since it uses as much as digits as its factorization.

21216421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 316730.

The product of its digits is 192, while the sum is 19.

The square root of 21216421 is about 4606.1286347648. The cubic root of 21216421 is about 276.8369408762.

Adding to 21216421 its reverse (12461212), we get a palindrome (33677633).

The spelling of 21216421 in words is "twenty-one million, two hundred sixteen thousand, four hundred twenty-one".