28021991 has 4 divisors (see below), whose sum is σ = 28497000. Its totient is φ = 27546984.

The previous prime is 28021969. The next prime is 28021993. The reversal of 28021991 is 19912082.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19912082 = 2 ⋅9956041.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-28021991 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (28021993) 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, 237416 + ... + 237533.

It is an arithmetic number, because the mean of its divisors is an integer number (7124250).

Almost surely, 2^28021991 is an apocalyptic number.

28021991 is a deficient number, since it is larger than the sum of its proper divisors (475009).

28021991 is an equidigital number, since it uses as much as digits as its factorization.

28021991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 475008.

The product of its (nonzero) digits is 2592, while the sum is 32.

The square root of 28021991 is about 5293.5801684682. The cubic root of 28021991 is about 303.7383735608.

The spelling of 28021991 in words is "twenty-eight million, twenty-one thousand, nine hundred ninety-one".