2561 has 4 divisors (see below), whose sum is σ = 2772. Its totient is φ = 2352.

The previous prime is 2557. The next prime is 2579. The reversal of 2561 is 1652.

2561 = T₁₃ + T₁₄ + ... + T₂₅.

2561 is nontrivially palindromic in base 14 and base 15.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 1600 + 961 = 40^2 + 31^2 .

It is a cyclic number.

It is not a de Polignac number, because 2561 - 2² = 2557 is a prime.

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

It is a Duffinian number.

2561 is an undulating number in base 14 and base 15.

It is a plaindrome in base 9.

It is not an unprimeable number, because it can be changed into a prime (2521) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 86 + ... + 111.

It is an arithmetic number, because the mean of its divisors is an integer number (693).

It is a Proth number, since it is equal to 5 ⋅ 2⁹ + 1 and 5 < 2⁹.

2²⁵⁶¹ is an apocalyptic number.

It is an amenable number.

2561 is a deficient number, since it is larger than the sum of its proper divisors (211).

2561 is a wasteful number, since it uses less digits than its factorization.

2561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 210.

The product of its digits is 60, while the sum is 14.

The square root of 2561 is about 50.6063237155. The cubic root of 2561 is about 13.6815885665.

Subtracting from 2561 its reverse (1652), we obtain a palindrome (909).

The spelling of 2561 in words is "two thousand, five hundred sixty-one".