21008639081 has 4 divisors (see below), whose sum is σ = 21018362964. Its totient is φ = 20998915200.

The previous prime is 21008639077. The next prime is 21008639107. The reversal of 21008639081 is 18093680012.

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 18230400400 + 2778238681 = 135020^2 + 52709^2 .

It is a cyclic number.

It is not a de Polignac number, because 21008639081 - 2² = 21008639077 is a prime.

It is a super-4 number, since 4×21008639081⁴ (a number of 42 digits) contains 4444 as substring. Note that it is a super-d number also for d = 2.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21008639011) 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, 4858700 + ... + 4863021.

It is an arithmetic number, because the mean of its divisors is an integer number (5254590741).

Almost surely, 2^21008639081 is an apocalyptic number.

It is an amenable number.

21008639081 is a deficient number, since it is larger than the sum of its proper divisors (9723883).

21008639081 is an equidigital number, since it uses as much as digits as its factorization.

21008639081 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9723882.

The product of its (nonzero) digits is 20736, while the sum is 38.

The spelling of 21008639081 in words is "twenty-one billion, eight million, six hundred thirty-nine thousand, eighty-one".