28308721 has 12 divisors (see below), whose sum is σ = 33178788. Its totient is φ = 24081792.

The previous prime is 28308713. The next prime is 28308733. The reversal of 28308721 is 12780382.

It can be written as a sum of positive squares in 2 ways, for example, as 21344400 + 6964321 = 4620^2 + 2639^2 .

It is not a de Polignac number, because 28308721 - 2³ = 28308713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28308521) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 4605 + ... + 8821.

It is an arithmetic number, because the mean of its divisors is an integer number (2764899).

Almost surely, 2^28308721 is an apocalyptic number.

It is an amenable number.

28308721 is a deficient number, since it is larger than the sum of its proper divisors (4870067).

28308721 is a wasteful number, since it uses less digits than its factorization.

28308721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4368 (or 4361 counting only the distinct ones).

The product of its (nonzero) digits is 5376, while the sum is 31.

The square root of 28308721 is about 5320.5940457810. The cubic root of 28308721 is about 304.7708428295.

The spelling of 28308721 in words is "twenty-eight million, three hundred eight thousand, seven hundred twenty-one".