21123581 has 4 divisors (see below), whose sum is σ = 21265500. Its totient is φ = 20981664.

The previous prime is 21123533. The next prime is 21123589. The reversal of 21123581 is 18532112.

It is a happy number.

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 8584900 + 12538681 = 2930^2 + 3541^2 .

It is a cyclic number.

It is not a de Polignac number, because 21123581 - 2⁶ = 21123517 is a prime.

It is a super-2 number, since 2×21123581² = 892411348527122, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21123589) 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, 70736 + ... + 71033.

It is an arithmetic number, because the mean of its divisors is an integer number (5316375).

Almost surely, 2^21123581 is an apocalyptic number.

It is an amenable number.

21123581 is a deficient number, since it is larger than the sum of its proper divisors (141919).

21123581 is a wasteful number, since it uses less digits than its factorization.

21123581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 141918.

The product of its digits is 480, while the sum is 23.

The square root of 21123581 is about 4596.0397082706. The cubic root of 21123581 is about 276.4325509347.

Adding to 21123581 its reverse (18532112), we get a palindrome (39655693).

The spelling of 21123581 in words is "twenty-one million, one hundred twenty-three thousand, five hundred eighty-one".