6561 has 9 divisors (see below), whose sum is σ = 9841. Its totient is φ = 4374.

The previous prime is 6553. The next prime is 6563. The reversal of 6561 is 1656.

6561 = T₈₀ + T₈₁.

The square root of 6561 is 81.

It is a perfect power (a square, a biquadrate, a 8-th power), and thus also a powerful number.

6561 is nontrivially palindromic in base 8.

It is a tau number, because it is divible by the number of its divisors (9).

It is not a de Polignac number, because 6561 - 2³ = 6553 is a prime.

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

It is a Duffinian number.

Its product of digits (180) is a multiple of the sum of its prime divisors (3).

It is a plaindrome in base 14.

It is a nialpdrome in base 3 and base 9.

It is not an unprimeable number, because it can be changed into a prime (6563) by changing a digit.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 2186 + 2187 + 2188.

2⁶⁵⁶¹ is an apocalyptic number.

6561 is the 81-st square number.

6561 is the 41-st centered octagonal number.

It is an amenable number.

6561 is a deficient number, since it is larger than the sum of its proper divisors (3280).

6561 is an frugal number, since it uses more digits than its factorization.

6561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24 (or 3 counting only the distinct ones).

The product of its digits is 180, while the sum is 18.

The cubic root of 6561 is about 18.7207544075.

The spelling of 6561 in words is "six thousand, five hundred sixty-one".