28091981 has 4 divisors (see below), whose sum is σ = 29060700. Its totient is φ = 27123264.

The previous prime is 28091977. The next prime is 28092023. The reversal of 28091981 is 18919082.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 19061956 + 9030025 = 4366^2 + 3005^2 .

It is a cyclic number.

It is not a de Polignac number, because 28091981 - 2² = 28091977 is a prime.

It is a super-2 number, since 2×28091981² = 1578318793008722, 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 (28091941) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 484316 + ... + 484373.

It is an arithmetic number, because the mean of its divisors is an integer number (7265175).

Almost surely, 2^28091981 is an apocalyptic number.

It is an amenable number.

28091981 is a deficient number, since it is larger than the sum of its proper divisors (968719).

28091981 is an equidigital number, since it uses as much as digits as its factorization.

28091981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 968718.

The product of its (nonzero) digits is 10368, while the sum is 38.

The square root of 28091981 is about 5300.1868834976. The cubic root of 28091981 is about 303.9910438555.

The spelling of 28091981 in words is "twenty-eight million, ninety-one thousand, nine hundred eighty-one".