Number reference
191 explained.
191: one hundred ninety-one, prime, palindromic prime; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- one hundred ninety-one
- Classification
- prime
- Factors
- 1, 191
- Prime factorization
- 191
- Square root
- 13.820275
- Cube root
- 5.758965
- Binary
- 10111111
- Octal
- 277
- Hexadecimal
- BF
- Roman numeral
- CXCI
- Digit sum
- 11
- Digital root
- 2
What makes 191 notable
- Prime. 191 is prime: it has exactly two divisors, 1 and itself, so it cannot be split into equal whole-number groups any other way.
- Palindromic prime. 191 is prime and reads the same forwards and backwards.
191 in everyday use
One hundred ninety-one opens the only prime quadruplet between 110 and 820: 191, 193, 197, and 199 are all prime, four primes packed into nine consecutive numbers, the tightest arrangement possible beyond the very smallest primes. Quadruplets like this appear as 11, 13, 17, 19 and then 101, 103, 107, 109 before reaching 191, and they are conjectured to go on forever, though nobody has proved it.
Taken alone, 191 is a palindromic prime and a Sophie Germain prime, because 2 × 191 + 1 = 383 is prime. Its reversal is itself, so the question of whether it is an emirp does not arise. It is the 43rd prime, and its digit sum is 11. Among three-digit palindromic primes it is followed only by 313, 353, 373, 383, 727, 757, 787, 797, 919, and 929.
Nearby numbers
How to read this page
Because 191 is prime, the factor list stops at 1 and 191 itself, so it cannot be shared into equal whole-number groups. That is also what makes it useful as a building block: every larger number with 191 in its prime factorisation inherits it from here.
Percentage calculator · Fraction calculator · Browse Number Explorer