81239805 has 24 divisors (see below), whose sum is σ = 141031800. Its totient is φ = 43261440.

The previous prime is 81239803. The next prime is 81239813. The reversal of 81239805 is 50893218.

It can be written as a sum of positive squares in 4 ways, for example, as 32421636 + 48818169 = 5694^2 + 6987^2 .

It is not a de Polignac number, because 81239805 - 2¹ = 81239803 is a prime.

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

It is a self number, because there is not a number n which added to its sum of digits gives 81239805.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81239803) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 46345 + ... + 48065.

It is an arithmetic number, because the mean of its divisors is an integer number (5876325).

Almost surely, 2^81239805 is an apocalyptic number.

It is an amenable number.

81239805 is a deficient number, since it is larger than the sum of its proper divisors (59791995).

81239805 is a wasteful number, since it uses less digits than its factorization.

81239805 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2781 (or 2778 counting only the distinct ones).

The product of its (nonzero) digits is 17280, while the sum is 36.

The square root of 81239805 is about 9013.3126540690. The cubic root of 81239805 is about 433.1014364098.

The spelling of 81239805 in words is "eighty-one million, two hundred thirty-nine thousand, eight hundred five".