81509209 has 4 divisors (see below), whose sum is σ = 83404816. Its totient is φ = 79613604.

The previous prime is 81509191. The next prime is 81509231. The reversal of 81509209 is 90290518.

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, and also an emirpimes, since its reverse is a distinct semiprime: 90290518 = 2 ⋅45145259.

It is a cyclic number.

It is not a de Polignac number, because 81509209 - 2¹⁹ = 80984921 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81509249) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 947739 + ... + 947824.

It is an arithmetic number, because the mean of its divisors is an integer number (20851204).

Almost surely, 2^81509209 is an apocalyptic number.

It is an amenable number.

81509209 is a deficient number, since it is larger than the sum of its proper divisors (1895607).

81509209 is a wasteful number, since it uses less digits than its factorization.

81509209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1895606.

The product of its (nonzero) digits is 6480, while the sum is 34.

The square root of 81509209 is about 9028.2450675643. The cubic root of 81509209 is about 433.5796523979.

It can be divided in two parts, 81509 and 209, that added together give a palindrome (81718).

The spelling of 81509209 in words is "eighty-one million, five hundred nine thousand, two hundred nine".