191

191 is a natural number that follows 190 and precedes 192. It is a prime number, making it indivisible by any positive integers other than 1 and itself. In addition to its primality, 191 is a palindromic prime, since it reads the same forward and backward in base 10. It is also a Sophie Germain prime, because 2 × 191 + 1 equals 383, which is itself prime.

In mathematical notation, 191 has several standard representations. In binary it is 10111111, in octal it is 277, in hexadecimal it is BF, and in Roman numerals it is written CXCI.

From a calendrical perspective, 191 is the 191st day of the year. In a common (non-leap) year, day 191 falls on July 10, while in a leap year it would be July 9.

Historically, the year 191 CE sits in the period of the Roman Empire during the reign of Emperor Commodus. It was a time characterized by imperial rule and ongoing regional dynamics across the Mediterranean world, though specific widely cited events for that exact year vary across sources.

In summary, 191 is notable in number theory for being a prime and a palindrome, with additional properties such as its status as a Sophie Germain prime. Its simple decimal form and distinct representations in other bases contribute to its usefulness as an example number in mathematics and related discussions.

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