28681721 has 4 divisors (see below), whose sum is σ = 28756992. Its totient is φ = 28606452.

The previous prime is 28681717. The next prime is 28681753. The reversal of 28681721 is 12718682.

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, and also an emirpimes, since its reverse is a distinct semiprime: 12718682 = 2 ⋅6359341.

It is a cyclic number.

It is not a de Polignac number, because 28681721 - 2² = 28681717 is a prime.

It is a super-2 number, since 2×28681721² = 1645282239043682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28681711) 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, 37061 + ... + 37826.

It is an arithmetic number, because the mean of its divisors is an integer number (7189248).

Almost surely, 2^28681721 is an apocalyptic number.

It is an amenable number.

28681721 is a deficient number, since it is larger than the sum of its proper divisors (75271).

28681721 is an equidigital number, since it uses as much as digits as its factorization.

28681721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 75270.

The product of its digits is 10752, while the sum is 35.

The square root of 28681721 is about 5355.5318129949. The cubic root of 28681721 is about 306.1035753724.

The spelling of 28681721 in words is "twenty-eight million, six hundred eighty-one thousand, seven hundred twenty-one".