1561 has 4 divisors (see below), whose sum is σ = 1792. Its totient is φ = 1332.

The previous prime is 1559. The next prime is 1567. The reversal of 1561 is 1651.

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 not a de Polignac number, because 1561 - 2¹ = 1559 is a prime.

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.

It is not an unprimeable number, because it can be changed into a prime (1567) by changing a digit.

It is a nontrivial repunit in base 39.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 105 + ... + 118.

It is an arithmetic number, because the mean of its divisors is an integer number (448).

2¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

1561 is a deficient number, since it is larger than the sum of its proper divisors (231).

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 product of its digits is 30, while the sum is 13.

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".