81807721 has 8 divisors (see below), whose sum is σ = 82534464. Its totient is φ = 81084240.

The previous prime is 81807709. The next prime is 81807767. The reversal of 81807721 is 12770818.

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 81807721 - 2²³ = 73419113 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81807701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 83766 + ... + 84736.

It is an arithmetic number, because the mean of its divisors is an integer number (10316808).

Almost surely, 2^81807721 is an apocalyptic number.

It is an amenable number.

81807721 is a deficient number, since it is larger than the sum of its proper divisors (726743).

81807721 is a wasteful number, since it uses less digits than its factorization.

81807721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1631.

The product of its (nonzero) digits is 6272, while the sum is 34.

The square root of 81807721 is about 9044.7620753672. The cubic root of 81807721 is about 434.1083086023.

It can be divided in two parts, 81807 and 721, that added together give a palindrome (82528).

The spelling of 81807721 in words is "eighty-one million, eight hundred seven thousand, seven hundred twenty-one".