21210043681 has 4 divisors (see below), whose sum is σ = 21218991664. Its totient is φ = 21201095700.

The previous prime is 21210043663. The next prime is 21210043697. The reversal of 21210043681 is 18634001212.

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 21210043681 - 2¹³ = 21210035489 is a prime.

It is a super-2 number, since 2×21210043681² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21210043601) 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, 4470435 + ... + 4475176.

It is an arithmetic number, because the mean of its divisors is an integer number (5304747916).

Almost surely, 2^21210043681 is an apocalyptic number.

It is an amenable number.

21210043681 is a deficient number, since it is larger than the sum of its proper divisors (8947983).

21210043681 is an equidigital number, since it uses as much as digits as its factorization.

21210043681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8947982.

The product of its (nonzero) digits is 2304, while the sum is 28.

Adding to 21210043681 its reverse (18634001212), we get a palindrome (39844044893).

The spelling of 21210043681 in words is "twenty-one billion, two hundred ten million, forty-three thousand, six hundred eighty-one".