28287781 has 4 divisors (see below), whose sum is σ = 28298700. Its totient is φ = 28276864.

The previous prime is 28287757. The next prime is 28287797. The reversal of 28287781 is 18778282.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 18778282 = 2 ⋅9389141.

It can be written as a sum of positive squares in 2 ways, for example, as 26687556 + 1600225 = 5166^2 + 1265^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-28287781 is a prime.

It is a super-2 number, since 2×28287781² = 1600397107807922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (28287481) 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, 885 + ... + 7573.

It is an arithmetic number, because the mean of its divisors is an integer number (7074675).

Almost surely, 2^28287781 is an apocalyptic number.

It is an amenable number.

28287781 is a deficient number, since it is larger than the sum of its proper divisors (10919).

28287781 is an equidigital number, since it uses as much as digits as its factorization.

28287781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10918.

The product of its digits is 100352, while the sum is 43.

The square root of 28287781 is about 5318.6258563655. The cubic root of 28287781 is about 304.6956778221.

It can be divided in two parts, 2828 and 7781, that added together give a square (10609 = 103²).

The spelling of 28287781 in words is "twenty-eight million, two hundred eighty-seven thousand, seven hundred eighty-one".