21510421 has 4 divisors (see below), whose sum is σ = 21916332. Its totient is φ = 21104512.

The previous prime is 21510407. The next prime is 21510427. The reversal of 21510421 is 12401512.

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 10692900 + 10817521 = 3270^2 + 3289^2 .

It is a cyclic number.

It is not a de Polignac number, because 21510421 - 2¹¹ = 21508373 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21510395 and 21510404.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21510427) 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, 202876 + ... + 202981.

It is an arithmetic number, because the mean of its divisors is an integer number (5479083).

Almost surely, 2^21510421 is an apocalyptic number.

It is an amenable number.

21510421 is a deficient number, since it is larger than the sum of its proper divisors (405911).

21510421 is an equidigital number, since it uses as much as digits as its factorization.

21510421 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 405910.

The product of its (nonzero) digits is 80, while the sum is 16.

The square root of 21510421 is about 4637.9328369436. The cubic root of 21510421 is about 278.1098068711.

Adding to 21510421 its reverse (12401512), we get a palindrome (33911933).

The spelling of 21510421 in words is "twenty-one million, five hundred ten thousand, four hundred twenty-one".