261 (two hundred ndsixty-one) is the

natural number In mathematics, the natural numbers are the numbers 0, 1, 2, 3, and so on, possibly excluding 0. Some start counting with 0, defining the natural numbers as the non-negative integers , while others start with 1, defining them as the positive in ...

following

260 __NOTOC__ Year 260 ( CCLX) was a leap year starting on Sunday of the Julian calendar. At the time, it was known as the Year of the Consulship of Saecularis and Donatus (or, less frequently, year 1013 ''Ab urbe condita ''Ab urbe condita' ...

and preceding 262. It has the prime factorization 3²·29.

In Mathematics

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

deficient number In number theory, a deficient number or defective number is a positive integer for which the sum of divisors of is less than . Equivalently, it is a number for which the sum of proper divisors (or aliquot sum) is less than . For example, th ...

, since 261 is nonagonal number, unique period in base 2, and the number of possible unfolded

tesseract In geometry, a tesseract or 4-cube is a four-dimensional hypercube, analogous to a two-dimensional square and a three-dimensional cube. Just as the perimeter of the square consists of four edges and the surface of the cube consists of six ...

patterns. 261 is a

lucky number In number theory, a lucky number is a natural number in a set which is generated by a certain " sieve". This sieve is similar to the sieve of Eratosthenes that generates the primes, but it eliminates numbers based on their position in the rema ...

, as well as an

odious number In number theory, an odious number is a positive integer that has an odd Hamming weight, number of 1s in its Binary number, binary expansion. Nonnegative integers that are not odious are called evil numbers. In computer science, an odious number ...

, meaning it has an odd number of 1's in its

binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical op ...

expansion, which is (with 3 ones in it).

References

{{Integers, 2 Integers