Number
1,991
1,991 is a composite number, odd, a calendar year.
Notable events — 1991 AD
- Jan 17 Operation Desert Storm begins as a US-led coalition launches airstrikes against Iraq.
- Jun 15 Mount Pinatubo erupts in the Philippines — the second-largest eruption of the 20th century.
- Jun 25 Slovenia and Croatia declare independence from Yugoslavia, beginning the Yugoslav Wars.
- Aug 19 Hardliners launch a failed coup against Soviet leader Mikhail Gorbachev.
- Dec 25 Gorbachev resigns and the Soviet Union is formally dissolved.
Events compiled from Wikipedia ↗ · Licensed CC BY-SA 4.0
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
-
Tuesday
January 1, 1991
- Ended on
-
Tuesday
December 31, 1991
- Friday the 13ths
-
2
2 Friday the 13ths this year.
- Easter Sunday
-
March 31
Sunday, March 31, 1991
- Decade
-
1990s
1990–1999
- Century
-
20th century
1901–2000
- Millennium
-
2nd millennium
1001–2000
- Years ago
-
35
35 years before 2026.
In other calendars
- Hebrew
-
5751 / 5752 AM
Rosh Hashanah falls in September/October.
- Islamic Hijri
-
1411 / 1412 AH
Lunar calendar; year spans differ from Gregorian.
- 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
-
2534 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Persian Solar Hijri
-
1369 / 1370 SH
Iranian calendar; Nowruz (new year) falls on the spring equinox.
- Ethiopian
-
1983 / 1984 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
1913 / 1912 Saka
Indian national calendar; year starts in March.
- Japanese
-
Heisei 3
Reign-era counting from the start of each emperor's reign.
Properties
- Parity
- Odd
- Digit count
- 4
- Digit sum
- 20
- Digit product
- 81
- Digital root
- 2
- Palindrome
- Yes
- Bit width
- 11 bits
- Flips to (rotate 180°)
- 1,661
- Recamán's sequence
- a(3,769) = 1,991
- Square (n²)
- 3,964,081
- Cube (n³)
- 7,892,485,271
- Divisor count
- 4
- σ(n) — sum of divisors
- 2,184
- φ(n) — Euler's totient
- 1,800
- Sum of prime factors
- 192
Primality
Prime factorization: 11 × 181
Divisors & multiples
Aliquot sum (sum of proper divisors):
193
First multiples
Sums & aliquot sequence
As consecutive integers:
995 + 996
176 + 177 + … + 186
80 + 81 + … + 101
Aliquot sequence:
1,991 → 193 → 1 → 0
— terminates at zero
Representations
- In words
- one thousand nine hundred ninety-one
- Ordinal
- 1991st
- Roman numeral
- MCMXCI
- Binary
- 11111000111
- Octal
- 3707
- Hexadecimal
- 0x7C7
- Base64
- B8c=
- One's complement
- 63,544 (16-bit)
In other bases
ternary (3)
2201202
quaternary (4)
133013
quinary (5)
30431
senary (6)
13115
septenary (7)
5543
nonary (9)
2652
undecimal (11)
1550
duodecimal (12)
119b
tridecimal (13)
ba2
tetradecimal (14)
a23
pentadecimal (15)
8cb
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒌋𒌋𒁹𒁹𒁹 𒌋𒁹
- Egyptian hieroglyphic
- 𓆼𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓏺
- Greek (Milesian)
- ͵αϡϟαʹ
- Mayan (base 20)
- 𝋤·𝋳·𝋫
- Chinese
- 一千九百九十一
- Chinese (financial)
- 壹仟玖佰玖拾壹
In other modern scripts
Eastern Arabic
١٩٩١
Devanagari
१९९१
Bengali
১৯৯১
Tamil
௧௯௯௧
Thai
๑๙๙๑
Tibetan
༡༩༩༡
Khmer
១៩៩១
Lao
໑໙໙໑
Burmese
၁၉၉၁
Digit at this position in famous constants
- π — Pi (π)
- Digit 1,991 = 4
- e — Euler's number (e)
- Digit 1,991 = 0
- φ — Golden ratio (φ)
- Digit 1,991 = 4
- √2 — Pythagoras's (√2)
- Digit 1,991 = 8
- ln 2 — Natural log of 2
- Digit 1,991 = 9
- γ — Euler-Mascheroni (γ)
- Digit 1,991 = 4
Also seen as
Unicode codepoint
߇
Nko Digit Seven
U+07C7
Decimal digit (Nd)
UTF-8 encoding: DF 87 (2 bytes).
Hex color
#0007C7
RGB(0, 7, 199)
IPv4 address
As an unsigned 32-bit integer, this is the IPv4 address 0.0.7.199.
- Address
- 0.0.7.199
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.7.199
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Position in π
The digit sequence 1991 first appears in π at position 8,355 of the decimal expansion (the 8,355ordinal-suffix:th digit after the integer 3).
Search range: the first 1,000,000 fractional digits of π. Any 6-digit-or-shorter string is virtually guaranteed to appear in there — the more interesting signal is the position.