81205509 has 32 divisors (see below), whose sum is σ = 136341504. Its totient is φ = 41762880.

The previous prime is 81205499. The next prime is 81205511. The reversal of 81205509 is 90550218.

It is not a de Polignac number, because 81205509 - 2⁴ = 81205493 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81205559) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 22087 + ... + 25499.

It is an arithmetic number, because the mean of its divisors is an integer number (4260672).

Almost surely, 2^81205509 is an apocalyptic number.

It is an amenable number.

81205509 is a deficient number, since it is larger than the sum of its proper divisors (55135995).

81205509 is a wasteful number, since it uses less digits than its factorization.

81205509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3537.

The product of its (nonzero) digits is 3600, while the sum is 30.

The square root of 81205509 is about 9011.4099340780. The cubic root of 81205509 is about 433.0404821458.

The spelling of 81205509 in words is "eighty-one million, two hundred five thousand, five hundred nine".