81509 has 2 divisors, whose sum is σ = 81510. Its totient is φ = 81508.

The previous prime is 81463. The next prime is 81517. The reversal of 81509 is 90518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 77284 + 4225 = 278^2 + 65^2 .

It is a cyclic number.

It is not a de Polignac number, because 81509 - 2¹⁶ = 15973 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 81509.

It is not a weakly prime, because it can be changed into another prime (81559) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40754 + 40755.

It is an arithmetic number, because the mean of its divisors is an integer number (40755).

2⁸¹⁵⁰⁹ is an apocalyptic number.

It is an amenable number.

81509 is a deficient number, since it is larger than the sum of its proper divisors (1).

81509 is an equidigital number, since it uses as much as digits as its factorization.

81509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 360, while the sum is 23.

The square root of 81509 is about 285.4978108498. The cubic root of 81509 is about 43.3579281813.

Subtracting 81509 from its reverse (90518), we obtain a palindrome (9009).

The spelling of 81509 in words is "eighty-one thousand, five hundred nine".