561 has 8 divisors (see below), whose sum is σ = 864. Its totient is φ = 320.

The previous prime is 557. The next prime is 563. The reversal of 561 is 165.

561 is nontrivially palindromic in base 2.

561 is a nontrivial binomial coefficient, being equal to C(34, 2).

It is a sphenic number, since it is the product of 3 distinct primes.

It is a 2-Lehmer number, since φ(561) divides (561-1)².

It is a cyclic number.

It is a Carmichael number.

It is not a de Polignac number, because 561 - 2² = 557 is a prime.

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

It is a Curzon number.

It is a plaindrome in base 6.

It is a nialpdrome in base 5.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (563) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 25 + ... + 41.

It is an arithmetic number, because the mean of its divisors is an integer number (108).

It is a Poulet number, since it divides 2⁵⁶⁰-1.

561 is a gapful number since it is divisible by the number (51) formed by its first and last digit.

561 is the 33-rd triangular number and also the 17-th hexagonal number.

It is an amenable number.

561 is a deficient number, since it is larger than the sum of its proper divisors (303).

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

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

The sum of its prime factors is 31.

The product of its digits is 30, while the sum is 12.

The square root of 561 is about 23.6854385647. The cubic root of 561 is about 8.2474739741.

It can be divided in two parts, 5 and 61, that added together give a palindrome (66).

The spelling of 561 in words is "five hundred sixty-one", and thus it is an aban number.