21006551509 has 4 divisors (see below), whose sum is σ = 21044265804. Its totient is φ = 20968837216.

The previous prime is 21006551507. The next prime is 21006551513. The reversal of 21006551509 is 90515560012.

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 2106351025 + 18900200484 = 45895^2 + 137478^2 .

It is a cyclic number.

It is not a de Polignac number, because 21006551509 - 2¹ = 21006551507 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21006551507) 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, 18856312 + ... + 18857425.

It is an arithmetic number, because the mean of its divisors is an integer number (5261066451).

Almost surely, 2^21006551509 is an apocalyptic number.

It is an amenable number.

21006551509 is a deficient number, since it is larger than the sum of its proper divisors (37714295).

21006551509 is an equidigital number, since it uses as much as digits as its factorization.

21006551509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 37714294.

The product of its (nonzero) digits is 13500, while the sum is 34.

The spelling of 21006551509 in words is "twenty-one billion, six million, five hundred fifty-one thousand, five hundred nine".