28991781 has 54 divisors (see below), whose sum is σ = 52885170. Its totient is φ = 15253056.

The previous prime is 28991737. The next prime is 28991791. The reversal of 28991781 is 18719982.

It can be written as a sum of positive squares in 3 ways, for example, as 540225 + 28451556 = 735^2 + 5334^2 .

It is not a de Polignac number, because 28991781 - 2¹⁰ = 28990757 is a prime.

It is a super-2 number, since 2×28991781² = 1681046731103922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (28991791) by changing a digit.

It is a polite number, since it can be written in 53 ways as a sum of consecutive naturals, for example, 74335 + ... + 74723.

It is an arithmetic number, because the mean of its divisors is an integer number (979355).

Almost surely, 2^28991781 is an apocalyptic number.

28991781 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

28991781 is a deficient number, since it is larger than the sum of its proper divisors (23893389).

28991781 is a wasteful number, since it uses less digits than its factorization.

28991781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 435 (or 412 counting only the distinct ones).

The product of its digits is 72576, while the sum is 45.

The square root of 28991781 is about 5384.4016380653. The cubic root of 28991781 is about 307.2026552607.

The spelling of 28991781 in words is "twenty-eight million, nine hundred ninety-one thousand, seven hundred eighty-one".