81729781 has 8 divisors (see below), whose sum is σ = 95930240. Its totient is φ = 68160528.

The previous prime is 81729719. The next prime is 81729803. The reversal of 81729781 is 18792718.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 81729781 - 2⁷ = 81729653 is a prime.

It is a super-2 number, since 2×81729781² = 13359514204615922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81729701) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 157521 + ... + 158038.

It is an arithmetic number, because the mean of its divisors is an integer number (11991280).

Almost surely, 2^81729781 is an apocalyptic number.

It is an amenable number.

81729781 is a deficient number, since it is larger than the sum of its proper divisors (14200459).

81729781 is a wasteful number, since it uses less digits than its factorization.

81729781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 315603.

The product of its digits is 56448, while the sum is 43.

The square root of 81729781 is about 9040.4524776142. The cubic root of 81729781 is about 433.9704033134.

The spelling of 81729781 in words is "eighty-one million, seven hundred twenty-nine thousand, seven hundred eighty-one".