28728721 has 4 divisors (see below), whose sum is σ = 32832832. Its totient is φ = 24624612.

The previous prime is 28728719. The next prime is 28728727. The reversal of 28728721 is 12782782.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 28728721 - 2¹ = 28728719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28728727) 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, 2052045 + ... + 2052058.

It is an arithmetic number, because the mean of its divisors is an integer number (8208208).

Almost surely, 2^28728721 is an apocalyptic number.

It is an amenable number.

28728721 is a deficient number, since it is larger than the sum of its proper divisors (4104111).

28728721 is an equidigital number, since it uses as much as digits as its factorization.

28728721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4104110.

The product of its digits is 25088, while the sum is 37.

The square root of 28728721 is about 5359.9180031042. The cubic root of 28728721 is about 306.2706854832.

The spelling of 28728721 in words is "twenty-eight million, seven hundred twenty-eight thousand, seven hundred twenty-one".