101 is the natural number following 100 and preceding 102.
In mathematics
101 is the 26th prime number and a palindromic number (therefore also a palindromic prime). The next prime is 103, with which it comprises a twin prime. Because the period length of its reciprocal is unique among primes, 101 is a unique prime.
101 is the sum of five consecutive primes (13 + 17 + 19 + 23 + 29). Given 101, the Mertens function returns 0.
101 is a centered decagonal number.
For a 3-digit number in base 10, this number has a relatively simple divisibility test. The 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 and 5, and it might not be terribly practical, but it is simpler than the divisibility tests for other 3-digit numbers.
In science
In astronomy,
- Messier object M101, a magnitude 8.5 spiral galaxy in the constellation Ursa Major, also known as the Pinwheel Galaxy
- The New General Catalogue object NGC101 , a magnitude 12.9 spiral galaxy in the constellation Sculptor
- Asteroid 101 Helena
- The Saros number of the solar eclipse series which began on 329 May 15 and ended on 1591 June 21. The duration of Saros series 101 was 1262.1 years, and it contained 71 solar eclipses.
- The Saros number of the lunar eclipse series which began on 342 September 21 and ended on 1839 February 28. The duration of Saros series 101 was 1496.5 years, and it contained 84 lunar eclipses.
In other fields
101 is also:
- The designation of US Highway 101, which runs from California to Washington.
- In universities, sometimes the course number of basic or entry-level courses. Also used in titles of books to indicate the book is meant for a beginner. At universities with four-digit course numbers, the equivalent course number is 1001 or 1010.
- In the title of the book The Hundred and One Dalmatians by Dodie Smith (and the various movie versions thereof).
- The number of keys on a standard PC alphanumeric keyboard, including 12 or so function keys, cursor keys, and a numeric keypads and various other additions to the QWERTY typewriter keyboard.
- 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'.
- Room 101 in George Orwell's novel Nineteen Eighty-Four
- Unit 101 of the Israeli Defense Forces.
- The year AD 101 or 101 BC.