28721561 has 4 divisors (see below), whose sum is σ = 31332624. Its totient is φ = 26110500.

The previous prime is 28721557. The next prime is 28721573. The reversal of 28721561 is 16512782.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 28721561 - 2² = 28721557 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28721501) 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, 1305515 + ... + 1305536.

It is an arithmetic number, because the mean of its divisors is an integer number (7833156).

Almost surely, 2^28721561 is an apocalyptic number.

It is an amenable number.

28721561 is a deficient number, since it is larger than the sum of its proper divisors (2611063).

28721561 is a wasteful number, since it uses less digits than its factorization.

28721561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2611062.

The product of its digits is 6720, while the sum is 32.

The square root of 28721561 is about 5359.2500408173. The cubic root of 28721561 is about 306.2452396317.

The spelling of 28721561 in words is "twenty-eight million, seven hundred twenty-one thousand, five hundred sixty-one".