81608721 has 16 divisors (see below), whose sum is σ = 115930944. Its totient is φ = 50886144.

The previous prime is 81608633. The next prime is 81608729. The reversal of 81608721 is 12780618.

It is a happy number.

It is not a de Polignac number, because 81608721 - 2¹¹ = 81606673 is a prime.

It is not an unprimeable number, because it can be changed into a prime (81608729) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 3405 + ... + 13221.

It is an arithmetic number, because the mean of its divisors is an integer number (7245684).

Almost surely, 2^81608721 is an apocalyptic number.

It is an amenable number.

81608721 is a deficient number, since it is larger than the sum of its proper divisors (34322223).

81608721 is a wasteful number, since it uses less digits than its factorization.

81608721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10000.

The product of its (nonzero) digits is 5376, while the sum is 33.

The square root of 81608721 is about 9033.7545350757. The cubic root of 81608721 is about 433.7560285077.

The spelling of 81608721 in words is "eighty-one million, six hundred eight thousand, seven hundred twenty-one".