101 (number)

101 (one hundred [and] one) is the natural number following 100 and preceding 102.
It is variously pronounced "one hundred and one" / "a hundred and one", "one hundred one" / "a hundred one", and "one oh one". As an ordinal number, 101st rather than 101th is the correct form.

Cardinal one hundred [and] one
Ordinal 101st
(one hundred [and] first)
Numeral system 101
Factorization prime
Prime 26th
Divisors 1, 101
Roman numeral CI
Binary 11001012
Octal 1458
Duodecimal 8512
Hexadecimal 6516


In mathematics

101 is the 26th prime number and a palindromic number (and so a palindromic prime). The next prime is 103, with which it makes a twin prime pair, making 101 a Chen prime. Because the period length of its reciprocal is unique among primes, 101 is a unique prime. 101 is an Eisenstein prime with no imaginary part and real part of the form 3n − 1.

101 is the sum of five consecutive primes (13 + 17 + 19 + 23 + 29). Given 101, the Mertens function returns 0. 101 is the fifth alternating factorial.

101 is a centered decagonal number.

The smallest prime for which 2^p-1 has not been completely factored.

For a 3-digit number in base 10, this number has a relatively simple divisibility test. The candidate number is split into groups of four, starting with the rightmost four, and added up to produce a 4-digit number. If this 4-digit number is of the form 1000a + 100b + 10a + b (where a and b are integers from 0 to 9), such as 3232 or 9797, or of the form 100b + b, such as 707 and 808, then the number is divisible by 101. This might not be as simple as the divisibility tests for numbers like 3 or 5, and it might not be terribly practical, but it is simpler than the divisibility tests for other 3-digit numbers.[citation needed]

On the seven-segment display of a calculator, 101 is both a strobogrammatic prime and a dihedral prime.

In science

  • In mineralogy, a Miller index of 101 is a crystal face that crosses the horizontal axis (a) and 3d vertical axis (c) but does not cross the 2d vertical axis (b).
  • In physics and chemistry, it is the atomic number of mendelevium, an actinide.

In books

According to Books in Print, more books are now published with a title that begins with '101' than '100'. They usually describe or discuss a list of items, such as 101 Ways to... or 101 Questions and Answers About... . This marketing tool is used to imply that the customer is given a little extra information beyond books that include only 100 items. Some books have taken this marketing scheme even further with titles that begin with '102', '103', or '1001'. The number is used in this context as a slang term when referring to "a 101 document" what is usually referred to as a statistical survey or overview of some topic.

Room 101 is a torture chamber in the novel Nineteen Eighty-Four by George Orwell.

In other fields

  • 101 Ranch Oil Company
  • Taipei 101, the tallest skyscraper in the world from 2004 to 2010.
  • 101st kilometre, a condition of release from the Gulag in the Soviet Union.
  • Roi Et Province, a province in Thailand. The name is literally 101 in Thai language.
  • An HTTP status code indicating that a client should switch protocols (e.g. to HTTPS).
  • The first course in a subject taught at a college or university in Australia, Canada, South Africa, or the United States.[1] By extension, "Topic 101" is used generally to indicate the basics of any subject. Used this way, it is always pronounced "one-oh-one".[2][3] See Course number.
  • For a new checking account in the US, the number of the first check.
  • A term used to define the number of keys on a computer keyboard[4]
  • An emergency telephone number in Argentina, Belarus, Belgium, India, and Israel.[citation needed]
  • 101 is the Single Non-Emergency Number (SNEN) in some parts of the UK, a telephone number used to call emergency services that are urgent but not emergencies.[5]
  • iCar 101 is a roadable aircraft design concept.[6]
  • 101 Dalmatians
  • The Zastava 101 is a compact car by the former Yugoslav automaker.
  • Highways numbered 101, the longest and most well-known of which is U.S. Route 101.
  • the number 101 has a very important symbolic meaning to the Western Literary Society VNICA, the oldest student society in Amsterdam
  • Rihanna - Rockstar 101
  • Vault 101 is the starting area of Fallout 3
  • Lebanese Red Cross EMS Center at Spears is coded 101
  • In gematria, a system of assigning numerical value to a word or phrase, 101 is the numerical value of the name מִיכָאֵל (Michael in English)
  • 101 is the identifying number of several infantry units in various militaries across the world, such as the American and Israeli paratrooper brigades


  1. ^ Engber, Daniel (6 September 2006), ""101 101: How did intro classes get their trademark number?", Slate, http://www.slate.com/id/2149082/ 
  2. ^ 101, Dictionary.com, http://dictionary.reference.com/search?q=101 
  3. ^ At universities with four-digit course numbers, the equivalent course number is 1001 or 1010. This use is unheard of in the United Kingdom.[citation needed]
  4. ^ 101-Key "Enhanced" Keyboard Layout, PC Guide, http://www.pcguide.com/ref/kb/layout/stdEnh101-c.html, retrieved 2009-05-04 
  5. ^ Welcome to 101, Home Office, http://www.101.gov.uk, retrieved 2009-04-05 
  6. ^ iCar 101 - The ultimate roadable aircraft, http://www.iCar-101.com, retrieved 2010-08-06 

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