21078581 has 4 divisors (see below), whose sum is σ = 22188000. Its totient is φ = 19969164.

The previous prime is 21078559. The next prime is 21078587. The reversal of 21078581 is 18587012.

It is a happy number.

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 21078581 - 2¹⁸ = 20816437 is a prime.

It is a super-2 number, since 2×21078581² = 888613153947122, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21078587) 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, 554681 + ... + 554718.

It is an arithmetic number, because the mean of its divisors is an integer number (5547000).

Almost surely, 2^21078581 is an apocalyptic number.

It is an amenable number.

21078581 is a deficient number, since it is larger than the sum of its proper divisors (1109419).

21078581 is a wasteful number, since it uses less digits than its factorization.

21078581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1109418.

The product of its (nonzero) digits is 4480, while the sum is 32.

The square root of 21078581 is about 4591.1415791718. The cubic root of 21078581 is about 276.2361147242.

The spelling of 21078581 in words is "twenty-one million, seventy-eight thousand, five hundred eighty-one".