191
191 is a prime, odd, a calendar year.
191 (one hundred ninety-one) is an odd 3-digit number. It is a prime number — divisible only by 1 and itself. Its digits read the same forwards and backwards, so it is a palindromic number. Written other ways, in Roman numerals it is CXCI and in binary, 10111111.
Interestingness
Historical context — 191 AD
Calendar year
Year 191 (CXCI) was a common year starting on Friday of the Julian calendar.
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Historical context — 191 BC
Calendar year
Year 191 BC was a year of the pre-Julian Roman calendar.
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Year facts
- Year type
-
Common year
Standard 365-day year; not divisible by 4 (or divisible by 100 but not 400).
- Days in year
- 365
- ISO weeks
- 52
- Started on
-
Saturday
January 1, 191
- Ended on
-
Saturday
December 31, 191
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
190s
190–199
- Century
-
2nd century
101–200
- Millennium
-
1st millennium
1–1000
- Years ago
-
1,835
1835 years before 2026.
In other calendars
- Hebrew
-
3951 / 3952 AM
Rosh Hashanah falls in September/October.
- Chinese
-
Year of the zodiac:Metal zodiac:Goat
Sexagenary cycle position 8 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
734 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
183 / 184 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
113 / 112 Saka
Indian national calendar; year starts in March.
Properties
Primality
191 is prime. It has exactly two divisors: 1 and itself.
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√191 = [13; (1, 4, 1, 1, 3, 2, 2, 13, 2, 2, 3, 1, 1, 4, 1, 26)]
Period length 16 — the block in parentheses repeats forever.
Representations
- In words
- one hundred ninety-one
- Ordinal
- 191st
- Roman numeral
- CXCI
- Binary
- 10111111
- Octal
- 277
- Hexadecimal
- 0xBF
- Base64
- vw==
- One's complement
- 64 (8-bit)
- Scientific notation
- 1.91 × 10²
- As a duration
- 191 s = 3 minutes, 11 seconds
As an angle
Historical numeral systems
- Babylonian (base 60)
- 𒁹𒁹𒁹 𒌋𒁹
- Egyptian hieroglyphic
- 𓍢𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓏺
- Greek (Milesian)
- ρϟαʹ
- Mayan (base 20)
- 𝋩·𝋫
- Chinese
- 一百九十一
- Chinese (financial)
- 壹佰玖拾壹
Digit at this position in famous constants
- π — Pi (π)
- Digit 191 = 9
- e — Euler's number (e)
- Digit 191 = 1
- φ — Golden ratio (φ)
- Digit 191 = 9
- √2 — Pythagoras's (√2)
- Digit 191 = 7
- ln 2 — Natural log of 2
- Digit 191 = 6
- γ — Euler-Mascheroni (γ)
- Digit 191 = 2
Also seen as
UTF-8 encoding: C2 BF (2 bytes).
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.191.
- Address
- 0.0.0.191
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.191
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Heard as a frequency, 191 Hz is closest to:
- Concert pitch (A4 = 440 Hz): G3 (196 Hz, -45¢ — about midway to F♯3)
- Scientific pitch (C4 = 256 Hz): G3 (191.8 Hz, -7¢)
- Baroque pitch (A4 = 415 Hz): G♯3 (195.9 Hz, -43¢)
Related reading
- Prime numbers — The building blocks of arithmetic: what primes are, why they matter, and how we find them.
- Mayan numerals — Vigesimal dots-and-bars with a shell zero — one of the earliest true zeros.