21042421 has 4 divisors (see below), whose sum is σ = 21059244. Its totient is φ = 21025600.

The previous prime is 21042419. The next prime is 21042449. The reversal of 21042421 is 12424012.

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

It can be written as a sum of positive squares in 2 ways, for example, as 3944196 + 17098225 = 1986^2 + 4135^2 .

It is a cyclic number.

It is not a de Polignac number, because 21042421 - 2¹ = 21042419 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21042395 and 21042404.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21042491) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6370 + ... + 9091.

It is an arithmetic number, because the mean of its divisors is an integer number (5264811).

Almost surely, 2^21042421 is an apocalyptic number.

It is an amenable number.

21042421 is a deficient number, since it is larger than the sum of its proper divisors (16823).

21042421 is a wasteful number, since it uses less digits than its factorization.

21042421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16822.

The product of its (nonzero) digits is 128, while the sum is 16.

The square root of 21042421 is about 4587.2018704217. The cubic root of 21042421 is about 276.0780646294.

Adding to 21042421 its reverse (12424012), we get a palindrome (33466433).

The spelling of 21042421 in words is "twenty-one million, forty-two thousand, four hundred twenty-one".