8561 has 4 divisors (see below), whose sum is σ = 9792. Its totient is φ = 7332.

The previous prime is 8543. The next prime is 8563. The reversal of 8561 is 1658.

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, and also an emirpimes, since its reverse is a distinct semiprime: 1658 = 2 ⋅829.

It is a cyclic number.

It is not a de Polignac number, because 8561 - 2¹⁰ = 7537 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a Curzon number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 8561.

It is not an unprimeable number, because it can be changed into a prime (8563) 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, 605 + ... + 618.

It is an arithmetic number, because the mean of its divisors is an integer number (2448).

2⁸⁵⁶¹ is an apocalyptic number.

It is an amenable number.

8561 is a deficient number, since it is larger than the sum of its proper divisors (1231).

8561 is a wasteful number, since it uses less digits than its factorization.

8561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1230.

The product of its digits is 240, while the sum is 20.

The square root of 8561 is about 92.5256721132. The cubic root of 8561 is about 20.4569789847.

Subtracting from 8561 its reverse (1658), we obtain a triangular number (6903 = T₁₁₇).

The spelling of 8561 in words is "eight thousand, five hundred sixty-one".