21585816509 has 2 divisors, whose sum is σ = 21585816510. Its totient is φ = 21585816508.

The previous prime is 21585816479. The next prime is 21585816583. The reversal of 21585816509 is 90561858512.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13902768100 + 7683048409 = 117910^2 + 87653^2 .

It is a cyclic number.

It is not a de Polignac number, because 21585816509 - 2²⁴ = 21569039293 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21585816599) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10792908254 + 10792908255.

It is an arithmetic number, because the mean of its divisors is an integer number (10792908255).

Almost surely, 2^21585816509 is an apocalyptic number.

It is an amenable number.

21585816509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21585816509 is an equidigital number, since it uses as much as digits as its factorization.

21585816509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 864000, while the sum is 50.

The spelling of 21585816509 in words is "twenty-one billion, five hundred eighty-five million, eight hundred sixteen thousand, five hundred nine".