28138721 has 8 divisors (see below), whose sum is σ = 30345000. Its totient is φ = 25938432.

The previous prime is 28138717. The next prime is 28138739. The reversal of 28138721 is 12783182.

It can be written as a sum of positive squares in 4 ways, for example, as 429025 + 27709696 = 655^2 + 5264^2 .

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

It is not a de Polignac number, because 28138721 - 2² = 28138717 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28138741) 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, 15371 + ... + 17103.

It is an arithmetic number, because the mean of its divisors is an integer number (3793125).

Almost surely, 2^28138721 is an apocalyptic number.

It is an amenable number.

28138721 is a deficient number, since it is larger than the sum of its proper divisors (2206279).

28138721 is a wasteful number, since it uses less digits than its factorization.

28138721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2995.

The product of its digits is 5376, while the sum is 32.

The square root of 28138721 is about 5304.5943294469. The cubic root of 28138721 is about 304.1595458978.

The spelling of 28138721 in words is "twenty-eight million, one hundred thirty-eight thousand, seven hundred twenty-one".