81131209 has 4 divisors (see below), whose sum is σ = 81161604. Its totient is φ = 81100816.

The previous prime is 81131201. The next prime is 81131221. The reversal of 81131209 is 90213118.

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 28249225 + 52881984 = 5315^2 + 7272^2 .

It is a cyclic number.

It is not a de Polignac number, because 81131209 - 2³ = 81131201 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81131201) 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, 10762 + ... + 16675.

It is an arithmetic number, because the mean of its divisors is an integer number (20290401).

Almost surely, 2^81131209 is an apocalyptic number.

It is an amenable number.

81131209 is a deficient number, since it is larger than the sum of its proper divisors (30395).

81131209 is a wasteful number, since it uses less digits than its factorization.

81131209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 30394.

The product of its (nonzero) digits is 432, while the sum is 25.

The square root of 81131209 is about 9007.2864393223. The cubic root of 81131209 is about 432.9083699031.

The spelling of 81131209 in words is "eighty-one million, one hundred thirty-one thousand, two hundred nine".