561 is nontrivially palindromic in base 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 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 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.
It is an amenable number.
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 square root of 561 is about 23.6854385647. The cubic root of 561 is about 8.2474739741.
The spelling of 561 in words is "five hundred sixty-one", and is thus an aban number.