261 has 6 divisors (see below), whose sum is σ = 390. Its totient is φ = 168.

The previous prime is 257. The next prime is 263. The reversal of 261 is 162.

261 is nontrivially palindromic in base 13.

It can be written as a sum of positive squares in only one way, i.e., 225 + 36 = 15^2 + 6^2 .

It is not a de Polignac number, because 261 - 2^{2} = 257 is a prime.

It is a super-3 number, since 3×261^{3} = 53338743, which contains 333 as substring.

It is a Harshad number since it is a multiple of its sum of digits (9), and also a Moran number because the ratio is a prime number: 29 = 261 / (2 + 6 + 1).

261 is an undulating number in base 13.

It is a Curzon number.

261 is a lucky number.

It is a plaindrome in base 6, base 12, base 14 and base 15.

It is a nialpdrome in base 7 and base 9.

It is a congruent number.

It is an inconsummate number, since it does not exist a number *n* which divided by its sum of digits gives 261.

It is not an unprimeable number, because it can be changed into a prime (263) 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 5 ways as a sum of consecutive naturals, for example, 6 + ... + 23.

It is an arithmetic number, because the mean of its divisors is an integer number (65).

261 is the 9-th nonagonal number.

It is an amenable number.

261 is a deficient number, since it is larger than the sum of its proper divisors (129).

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

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

The sum of its prime factors is 35 (or 32 counting only the distinct ones).

The product of its digits is 12, while the sum is 9.

The square root of 261 is about 16.1554944214. The cubic root of 261 is about 6.3906765284.

The spelling of 261 in words is "two hundred sixty-one", and is thus an aban number.

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