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 (one hundred [and] first), rather than 101th, is the correct form.

Look up one hundred and one or one hundred one in Wiktionary, the free dictionary.

In mathematics

101 is:

• the 26th prime number and the smallest above 100.
• a palindromic number in decimal, and so a palindromic prime.
• a Chen prime since 103 is also prime, with which it makes a twin prime pair.
• a sexy prime since 107 and 113 are also prime, with which it makes a sexy prime triplet.
• a unique prime because the period length of its reciprocal is unique among primes.
• an Eisenstein prime with no imaginary part and real part of the form ${\displaystyle 3n-1}$.
• the fifth alternating factorial.^[1]
• a centered decagonal number.^[2]
• the only existing prime with alternating 1s and 0s in decimal and the largest known prime of the form ${\displaystyle 10^{n}+1}$.^[3]
• the number of compositions of 12 into distinct parts.^[4]
• the smallest number that can be expressed as the sum of three distinct nonzero squares in more than two ways: ${\displaystyle 9^{2}+4^{2}+2^{2))$, ${\displaystyle 8^{2}+6^{2}+1^{2))$ or ${\displaystyle 7^{2}+6^{2}+4^{2))$ (see image).^[5][6]

Given 101, the Mertens function returns 0.^[7] It is the second prime to have this property after 2.^[8]

For a 3-digit number in decimal, 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 ${\displaystyle 1000a+100b+10a+b}$ (where a and b are integers from 0 to 9), such as 3232 or 9797, or of the form ${\displaystyle 100b+b}$, such as 707 and 808, then the number is divisible by 101.^[9]

On the seven-segment display of a calculator, 101 is both a strobogrammatic prime and a dihedral prime.^[10]

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 astronomy it is the Messier designation given to the Pinwheel Galaxy in Ursa Major.

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 education

In American university course numbering systems, the number 101 is often used for an introductory course at a beginner's level in a department's subject area.^[11][12][13] This common numbering system was designed to make transfer between colleges easier. It can also indicate a course for students not intending to major in the subject; e.g. a student intending to major in English would take English 111 not English 101.

In theory, any numbered course in one academic institution should bring a student to the same standard as a similarly numbered course at other institutions.^[12] One of earliest such usages, perhaps the first, was by the University of Buffalo in 1929.^[12][13]

Based on this usage, the term "101" (pronounced ONE-oh-ONE) has gained a slang sense referring to basic knowledge of a topic or a collection of introductory materials to a topic, as in the sentence, "Boiling potatoes is Cooking 101".^[13] The Oxford English Dictionary records the usage of "101" in this slang sense from 1986.^[13]

In other fields

In public life:

• In Hinduism, 101 is a lucky number.
• Bill 101, Charter of the French Language in Quebec, Canada.
• 101st kilometre, a condition of release from the Gulag in the Soviet Union.
• 101 is the main Police Emergency Number in Belgium.
• 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. 101 is now available across all areas of England and Wales.^[14][15]
• Highways numbered 101, the longest and most well-known of which are BR-101 and U.S. Route 101.
• The number of the first check for a new checking account in the US.
• Lebanese Red Cross EMS Center at Spears is coded 101.

In construction and technics:

• Taipei 101, the tallest skyscraper in the world from 2004 to 2010.
• An HTTP status code indicating that the server is switching protocols as requested by the client to do so.
• A term used to define the number of keys on a standard 101 key computer keyboard^[16]
• iCar 101 is a roadable aircraft design concept.^[17][18]
• The Zastava 101 is a compact car by the former Yugoslav automaker.

Proper names:

• Roi Et Province, a province in Thailand. The name is literally 101 in Thai language.
• 101 Ranch Oil Company
• "l'ordre des Piliers du 101" is an important student association in Belgium since 1977.

In entertainment:

• 101, a Depeche Mode album
• 101 Dalmatians, a 1956 children's novel by Dodie Smith
• 101 Reykjavík, a 2000 Icelandic film directed by Baltasar Kormákur, 101 being a postcode of Reykjavik
• The apartment number of Neo in The Matrix
• Vault 101 is the starting area of the 2008 action role-playing game Fallout 3
• The Wonderful 101, an action-adventure game for the Wii U
• Produce 101, a Korean reality girl group survival show
• TV show Zoey 101
• The house number of Samson en Gert in the eponymous Belgian kids' television show.

Others:

• 101 is the identifying number of several infantry units in various militaries across the world, such as the American and Israeli paratrooper brigades.
• 101 was the tail number of a Polish Air Force Tu-154, which crashed on 10 April 2010 whilst on its final approach to Smolensk North Airport killing all aboard including president Lech Kaczyński and his wife. See 2010 Polish Air Force Tu-154 crash.