191 (number)
{{Infobox number
| number = 191
| prime = 43rd
}}
191 (one hundred [and] ninety-one) is the natural number following 190 and preceding 192.
In mathematics
191 is a prime number, part of a prime quadruplet of four primes: 191, 193, 197, and 199.{{Cite OEIS|A136162|List of prime quadruplets {p, p+2, p+6, p+8} }} Because doubling and adding one produces another prime number (383), 191 is a Sophie Germain prime.{{Cite OEIS|A005384|Sophie Germain primes p: 2p+1 is also prime}} It is the smallest prime that is not a full repetend prime in any base from 2 to 10; in fact, the smallest base for which 191 is a full period prime is base 19.Wolfram MathWorld; [http://mathworld.wolfram.com/PrimitiveRoot.html Primitive Root]
See also
References
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