81728981 has 4 divisors (see below), whose sum is σ = 81813612. Its totient is φ = 81644352.

The previous prime is 81728971. The next prime is 81728987. The reversal of 81728981 is 18982718.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 18982718 = 2 ⋅9491359.

It can be written as a sum of positive squares in 2 ways, for example, as 31460881 + 50268100 = 5609^2 + 7090^2 .

It is a cyclic number.

It is not a de Polignac number, because 81728981 - 2⁶ = 81728917 is a prime.

It is a super-2 number, since 2×81728981² = 13359252670596722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81728987) 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, 40850 + ... + 42803.

It is an arithmetic number, because the mean of its divisors is an integer number (20453403).

Almost surely, 2^81728981 is an apocalyptic number.

It is an amenable number.

81728981 is a deficient number, since it is larger than the sum of its proper divisors (84631).

81728981 is an equidigital number, since it uses as much as digits as its factorization.

81728981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 84630.

The product of its digits is 64512, while the sum is 44.

The square root of 81728981 is about 9040.4082319329. The cubic root of 81728981 is about 433.9689873569.

The spelling of 81728981 in words is "eighty-one million, seven hundred twenty-eight thousand, nine hundred eighty-one".