191 (number): Difference between revisions - Wikipedia

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{{Infobox number

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| prime = yes43rd

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'''191''' ('''one hundred [and] ninety-one''') is the [[natural number]] following [[190 (number)|190]] and preceding [[192 (number)|192]].

==In mathematics==

191 is a [[prime number]], part of a [[prime quadruplet]] of four [[prime number|primes]]: 191, [[193 (number)|193]], [[197 (number)|197]], and [[199 (number)|199]].<ref>{{Cite OEIS|A136162|List of prime quadruplets {p, p+2, p+6, p+8} }}</ref> Because doubling and adding one produces another prime number (383), 191 is a [[Sophie Germain prime]].<ref>{{Cite OEIS|A005384|Sophie Germain primes p: 2p+1 is also prime}}</ref> It is the smallest prime that is not a [[full reptend prime]] in ''any'' base from 2 to 10 –; in fact, the smallest base for which 191 is a full period prime is [[base 19]].<ref>Wolfram MathWorld; [http://mathworld.wolfram.com/PrimitiveRoot.html Primitive Root]</ref>

==See also==

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[[Category:Integers]]

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