21004218581 has 4 divisors (see below), whose sum is σ = 21005791500. Its totient is φ = 21002645664.

The previous prime is 21004218551. The next prime is 21004218601. The reversal of 21004218581 is 18581240012.

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 17500644100 + 3503574481 = 132290^2 + 59191^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21004218581 is a prime.

It is a super-2 number, since 2×21004218581² (a number of 21 digits) 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 (21004218541) 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, 766256 + ... + 793193.

It is an arithmetic number, because the mean of its divisors is an integer number (5251447875).

Almost surely, 2^21004218581 is an apocalyptic number.

It is an amenable number.

21004218581 is a deficient number, since it is larger than the sum of its proper divisors (1572919).

21004218581 is a wasteful number, since it uses less digits than its factorization.

21004218581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1572918.

The product of its (nonzero) digits is 5120, while the sum is 32.

Adding to 21004218581 its reverse (18581240012), we get a palindrome (39585458593).

The spelling of 21004218581 in words is "twenty-one billion, four million, two hundred eighteen thousand, five hundred eighty-one".