261 (two hundred ndsixty-one) is a natural number proceeded by the number

260 __NOTOC__ Year 260 ( CCLX) was a leap year starting on Sunday (link will display the full calendar) 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 ...

and followed by

262 __NOTOC__ Year 262 ( CCLXII) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Gallienus and Faustianus (or, less frequently, year ...

. 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 In number theory, a deficient number or defective number is a number ''n'' for which the sum of divisors of ''n'' is less than 2''n''. Equivalently, it is a number for which the sum of proper divisors (or aliquot sum) is less than ''n''. For ex ...

, since 261 is

nonagonal number A nonagonal number (or an enneagonal number) is a figurate number that extends the concept of triangular and square numbers to the nonagon (a nine-sided polygon). However, unlike the triangular and square numbers, the patterns involved in the constr ...

, unique period in base 2, and the number of possible unfolded

tesseract In geometry, a tesseract is the four-dimensional analogue of the cube; the tesseract is to the cube as the cube is to the square. Just as the surface of the cube consists of six square faces, the hypersurface of the tesseract consists of e ...

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 remain ...

, as well as 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 digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...

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

In other fields

* 261 AD, a calendar year * 261 AM, a radio frequency * 261 BC, a calendar year * 261 Fearless, a non-profit organization started by

Kathrine Switzer Kathrine Virginia Switzer (born January 5, 1947) is an American marathon runner, author, and television commentator. In 1967, she became the first woman to run the Boston Marathon as an officially registered competitor. During her run, race man ...

, who in 1967 wore bib number 261 when she became the first woman to run the Boston Marathon as a numbered entrant * +261, a country calling code; see

telephone numbers in Madagascar Telephone numbers in Madagascar are seven digits long, the first two digits being an area code. Recent ITU documents suggest the area code is preceded by a two digit operator code, for example "20" for Telecom Malagasy. International numbering ...

H.261 H.261 is an ITU-T video compression standard, first ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Study Group 16 Video Coding Experts Group (VCEG, then Specialists Gro ...

, a video compression standard

Military / Transportation

* Flight 261, several airliner incidents * German submarine U-261, which was sunk in 1942 *

KiHa 261 series The is a tilting train, tilting diesel multiple unit (DMU) train type operated by Hokkaido Railway Company (JR Hokkaido) on limited express services in Hokkaido, Japan, since 2000. Design The running gear was based on the KiHa 201 series trai ...

, a type of diesel train in Japan * Messerschmitt Me 261, an airplane of the late 1930s *

Milwaukee Road 261 Milwaukee Road 261 is a class "S3" 4-8-4 "Northern" type steam locomotive built by the American Locomotive Company (ALCO) in Schenectady, New York in July, 1944 for the Milwaukee Road. It was used for heavy mainline freight and passenger work ...

, a restored steam locomotive built in 1944 * No. 261 Squadron RAF, of World War I and World War II *

VMM-261 Marine Medium Tiltrotor Squadron 261 (VMM-261) is a United States Marine Corps tiltrotor squadron consisting of MV-22 Osprey transport aircraft. The squadron, known as the "Raging Bulls", is based at Marine Corps Air Station (MCAS) New River, No ...

, a United States Marine Corps squadron ;Space * 261 Prymno, a minor planet * Kosmos 261, a Soviet satellite launched in 1968 * NGC 261, a diffuse nebula located in the constellation Tucana ;See also * List of highways numbered 261 *

References

{{Integers, 2 Integers