181 (number)

181 (one hundred [and] eighty-one) is the natural number following 180 and preceding 182.

In mathematics

181 is an odd number
181 is a centered number
- 181 is a centered pentagonal number^[1]
- 181 is a centered 12-gonal number^[2]
- 181 is a centered 18-gonal number^[3]
- 181 is a centered 30-gonal number
- 181 is a centered square number^[4]
- 181 is a star number that represents a centered hexagram (as in the game of Chinese checkers)
181 is a deficient number, as 1 is less than 181
181 is an odious number
181 is a prime number
- 181 is a Chen prime^[5]
- 181 is a dihedral prime
- 181 is a full reptend prime^[6]
- 181 is a palindromic prime
- 181 is a strobogrammatic prime, the same when viewed upside down
- 181 is a twin prime with 179
181 is a square-free number
181 is an undulating number, if written in the ternary, the negaternary, or the nonary numeral systems
181 is the difference of 2 square numbers: 91² – 90²
181 is the sum of 2 consecutive square numbers: 9² + 10²
181 is the sum of 5 consecutive prime numbers: 29 + 31 + 37 + 41 + 43

181st (Brandon) Battalion, CEF was a unit in the Canadian Expeditionary Force during World War I
181st Airlift Squadron is a unit of the Texas Air National Guard
181st Infantry Brigade of the United States Army based at Fort McCoy, Wisconsin
181st Intelligence Wing is a unit of the United States Air Force located at Hulman Field, Terre Haute, Indiana
AN/APQ-181 an all-weather, low probability of intercept (LPI) radar system for the U.S. Air Force B-2A Spirit bomber aircraft
Bücker Bü 181 Bestmann single-engine trainer aircraft during World War II
USNS Adelphi (T-AG-181) was a ship scheduled to be acquired by the United States Navy, however, the program was cancelled
USNS Fred C. Ainsworth (T-AP-181) was a United States Navy troop transport during World War II
USNS Potomac (T-AO-181) was a United States Navy oiler following the Vietnam War
USS Accokeek (ATA-181) was a United States Navy ATA-174-class auxiliary ocean tugboat during World War II
USS Flagler (AK-181) was a United States Navy Alamosa-class cargo ship following World War II
USS Hopewell (DD-181) was a United States Navy Template:Sclass- following World War I
USS Pompano (SS-181) was a United States Navy Porpoise-class submarine during World War II
USS Straub (DE-181) was a United States Navy Template:Sclass- during World War II

The Volkswagen 181
Lufthansa Flight 181, which was hijacked on October 13, 1977
London Buses route 181
181 Union City-New York, a New Jersey Transit bus route from New Jersey to New York
The Córas Iompair Éireann CIE 181 Class diesel locomotives, and numbered B181 to B192, built by General Motors Electro-Motive Division (EMD) in 1966
East 181st Avenue, a light rail station on the MAX Blue Line in Gresham, Oregon
New York City Subway stations in Manhattan:
- 181st Street (IND Eighth Avenue Line), at Fort Washington Avenue served by the A train
- 181st Street (IRT Broadway – Seventh Avenue Line), at St. Nicholas Avenue served by the 1 train
181st Street (Manhattan)
Oregon Route 229, also known as the Siletz Highway No. 181

181 is also:

^ "Sloane's A005891 : Centered pentagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
^ "Sloane's A003154 : Centered 12-gonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
^ "Sloane's A069131 : Centered 18-gonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
^ "Sloane's A001844 : Centered square numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
^ "Sloane's A109611 : Chen primes". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
^ "Sloane's A001913 : Full reptend primes". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.