61 (number)
From Halbeeg, the open encyclopedia · Af-Soomaali
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61 is a natural number that lies between 60 and 62. In decimal notation it is written as six tens and one. It functions both as a cardinal number—used for counting—and as an ordinal number, described as "sixty-first".
In number theory, 61 is classified as a prime number, meaning it is divisible only by 1 and itself. Its only divisors are therefore 1 and 61.
Mathematical properties
61 is an odd number and a prime number. It is the eighteenth prime number in the sequence beginning with 2 (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61). The sum of its digits is 6 + 1 = 7.
61 is one of a pair of twin primes when considered alongside 59, since their difference is two. It also appears in various contexts of number theory, such as in problems involving Diophantine equations, particularly in the Pell equation x² − 61y² = 1, whose smallest positive solution is notably large compared to other equations of this form.
Notation and numeral systems
In Roman numerals, 61 is written as LXI. In binary notation it is 111101. Different languages have their own forms for naming this number: in Somali "lixdan iyo kow", in English "sixty-one", in Arabic "واحد وستون" (wāḥid wa-sittūn).
Other uses
61 serves as a designation or numerical reference in various contexts: a year in history, a vehicle or road number, a player number in sports, and an international country calling code (Australia uses 61 as its international dialing code).