21042561 has 4 divisors (see below), whose sum is σ = 28056752. Its totient is φ = 14028372.

The previous prime is 21042551. The next prime is 21042563. The reversal of 21042561 is 16524012.

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 not a de Polignac number, because 21042561 - 2¹³ = 21034369 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21042563) 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, 3507091 + ... + 3507096.

It is an arithmetic number, because the mean of its divisors is an integer number (7014188).

Almost surely, 2^21042561 is an apocalyptic number.

It is an amenable number.

21042561 is a deficient number, since it is larger than the sum of its proper divisors (7014191).

21042561 is an equidigital number, since it uses as much as digits as its factorization.

21042561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7014190.

The product of its (nonzero) digits is 480, while the sum is 21.

The square root of 21042561 is about 4587.2171302436. The cubic root of 21042561 is about 276.0786768980.

Adding to 21042561 its reverse (16524012), we get a palindrome (37566573).

The spelling of 21042561 in words is "twenty-one million, forty-two thousand, five hundred sixty-one".