101 (number)

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← 100 101 102 →
Cardinalone hundred [and] one
(one hundred [and] first)
Divisors1, 101
Greek numeralΡΑ´
Roman numeralCI

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.

In mathematics[edit]

101 is:

Given 101, the Mertens function returns 0.[4] It is the second prime having this property.[5]

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.[6]

On the seven-segment display of a calculator, 101 is both a strobogrammatic prime and a dihedral prime.[7]

In science[edit]

  • 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[edit]

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.

Creative Writing 101 by Raymond Carver, "A writer's values and craft. This was what the man (John Gardner) taught and what he stood for, and this is what I've kept by me in the years since that brief but all important time."

In education[edit]

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. This common numbering system was designed to make transfer between colleges easier. 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.[8] The term was first introduced by the University of Buffalo in 1929.[9]

Based on this usage, the term "101" has been extended to mean an introductory level of learning or a collection of introductory materials to a topic.

In other fields[edit]


  • Wells, D. The Penguin Dictionary of Curious and Interesting Numbers London: Penguin Group. (1987): page 133. ISBN 978-0-14-008029-2
  1. ^ "Sloane's A005165 : Alternating factorials". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 27 May 2016.
  2. ^ "Sloane's A062786 : Centered 10-gonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 27 May 2016.
  3. ^ Prime Curios! 101
  4. ^ "Sloane's A028442 : Numbers n such that Mertens' function is zero". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 27 May 2016.
  5. ^ "Sloane's A100669 : Zeros of the Mertens function that are also prime". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 29 May 2016.
  6. ^ Renault, Marc (November 2006), "Stupid Divisibility Tricks 101 Ways to Stupefy Your Friends", Math Horizons, 14 (2): 18–21, 42, doi:10.1080/10724117.2006.11974676, JSTOR 25678653, S2CID 125269086
  7. ^ "Sloane's A134996 : Dihedral calculator primes: p, p upside down, p in a mirror, p upside-down-and-in-a-mirror are all primes". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 17 December 2020.
  8. ^ Forest, J.J.F. (2002) Higher education in the United States: an encyclopedia p.73. ABC-CLIO. ISBN 1-57607-248-7. Retrieved October 2011
  9. ^ Engber, Daniel (6 September 2006). "101 101". Slate. Retrieved 9 May 2017.
  10. ^ Kozierok, Charles. "101-Key "Enhanced" Keyboard Layout". The PC Guide. Retrieved 29 May 2019.
  11. ^ "Report a crime or antisocial behaviour - GOV.UK". www.direct.gov.uk. Retrieved 4 April 2018.
  12. ^ Welcome to 101, Home Office, retrieved 5 April 2009
  13. ^ iCar 101 - The ultimate roadable aircraft, retrieved 6 August 2010