261 (number)
261 (two hundred [and] sixty-one) is a natural number proceeded by the number 260 and followed by 262. It has the prime factorization 32·29.

Mathematical properties 
There are six divisors of this number, the divisors being , , , , , and 261 itself. 261 is a deficient number, since 

261 is nonagonal number, unique period in base 2, and the number of possible unfolded tesseract patterns.

261 is a lucky number, as well as an odious number, meaning it has an odd number of 1's in its binary expansion, which is  (with 3 ones in it).

References 

Integers