1561 is nontrivially palindromic in base 14.
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: 1651 = 13 ⋅127.
It is a cyclic number.
It is the 40-th Hogben number.
1561 is an undulating number in base 14.
It is a plaindrome in base 11.
It is a nialpdrome in base 5, base 12 and base 13.
It is a congruent number.
21561 is an apocalyptic number.
It is an amenable number.
1561 is an equidigital number, since it uses as much as digits as its factorization.
1561 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 230.
The square root of 1561 is about 39.5094925303. The cubic root of 1561 is about 11.6002576241.
The spelling of 1561 in words is "one thousand, five hundred sixty-one".