28001991 has 16 divisors (see below), whose sum is σ = 39670400. Its totient is φ = 17519328.

The previous prime is 28001977. The next prime is 28001993. The reversal of 28001991 is 19910082.

It is not a de Polignac number, because 28001991 - 2⁶ = 28001927 is a prime.

It is a super-2 number, since 2×28001991² = 1568222999928162, 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 (28001993) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 3960 + ... + 8466.

It is an arithmetic number, because the mean of its divisors is an integer number (2479400).

Almost surely, 2^28001991 is an apocalyptic number.

28001991 is a deficient number, since it is larger than the sum of its proper divisors (11668409).

28001991 is a wasteful number, since it uses less digits than its factorization.

28001991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4638.

The product of its (nonzero) digits is 1296, while the sum is 30.

The square root of 28001991 is about 5291.6907506014. The cubic root of 28001991 is about 303.6660944558. Note that the first 3 decimals are identical.

The spelling of 28001991 in words is "twenty-eight million, one thousand, nine hundred ninety-one".