28131721 has 4 divisors (see below), whose sum is σ = 28252692. Its totient is φ = 28010752.

The previous prime is 28131713. The next prime is 28131757. The reversal of 28131721 is 12713182.

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: 12713182 = 2 ⋅6356591.

It can be written as a sum of positive squares in 2 ways, for example, as 16646400 + 11485321 = 4080^2 + 3389^2 .

It is a cyclic number.

It is not a de Polignac number, because 28131721 - 2³ = 28131713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28131121) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 60136 + ... + 60601.

It is an arithmetic number, because the mean of its divisors is an integer number (7063173).

Almost surely, 2^28131721 is an apocalyptic number.

It is an amenable number.

28131721 is a deficient number, since it is larger than the sum of its proper divisors (120971).

28131721 is a wasteful number, since it uses less digits than its factorization.

28131721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 120970.

The product of its digits is 672, while the sum is 25.

The square root of 28131721 is about 5303.9344830041. The cubic root of 28131721 is about 304.1343221335.

The spelling of 28131721 in words is "twenty-eight million, one hundred thirty-one thousand, seven hundred twenty-one".