28921221 has 18 divisors (see below), whose sum is σ = 48596262. Its totient is φ = 16526160.

The previous prime is 28921219. The next prime is 28921223. The reversal of 28921221 is 12212982.

It is an interprime number because it is at equal distance from previous prime (28921219) and next prime (28921223).

It can be written as a sum of positive squares in only one way, i.e., 16769025 + 12152196 = 4095^2 + 3486^2 .

It is not a de Polignac number, because 28921221 - 2¹ = 28921219 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (28921223) 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 17 ways as a sum of consecutive naturals, for example, 32350 + ... + 33231.

Almost surely, 2^28921221 is an apocalyptic number.

28921221 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

28921221 is a deficient number, since it is larger than the sum of its proper divisors (19675041).

28921221 is a wasteful number, since it uses less digits than its factorization.

28921221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 65601 (or 65591 counting only the distinct ones).

The product of its digits is 1152, while the sum is 27.

The square root of 28921221 is about 5377.8453863978. The cubic root of 28921221 is about 306.9532302217.

The spelling of 28921221 in words is "twenty-eight million, nine hundred twenty-one thousand, two hundred twenty-one".