81971581 has 4 divisors (see below), whose sum is σ = 82892700. Its totient is φ = 81050464.

The previous prime is 81971579. The next prime is 81971629. The reversal of 81971581 is 18517918.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 18517918 = 2 ⋅9258959.

It can be written as a sum of positive squares in 2 ways, for example, as 7425625 + 74545956 = 2725^2 + 8634^2 .

It is a cyclic number.

It is not a de Polignac number, because 81971581 - 2¹ = 81971579 is a prime.

It is a super-2 number, since 2×81971581² = 13438680183279122, which contains 22 as substring.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81971521) 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, 460426 + ... + 460603.

It is an arithmetic number, because the mean of its divisors is an integer number (20723175).

Almost surely, 2^81971581 is an apocalyptic number.

It is an amenable number.

81971581 is a deficient number, since it is larger than the sum of its proper divisors (921119).

81971581 is an equidigital number, since it uses as much as digits as its factorization.

81971581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 921118.

The product of its digits is 20160, while the sum is 40.

The square root of 81971581 is about 9053.8158253854. The cubic root of 81971581 is about 434.3979534202.

The spelling of 81971581 in words is "eighty-one million, nine hundred seventy-one thousand, five hundred eighty-one".