11
11 is a prime, odd, a calendar year.
11 (eleven) is an odd 2-digit number. It is a prime number — divisible only by 1 and itself. It is a Lucas number. Its digits read the same forwards and backwards, so it is a palindromic number. Written other ways, in Roman numerals it is XI and in binary, 1011.
Interestingness
Historical context — 11 AD
Calendar year
AD 11 (XI) was a common year starting on Thursday of the Julian calendar.
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Historical context — 11 BC
Calendar year
Year 11 BC was either a common year starting on Monday or Tuesday or a leap year starting on Sunday, Monday or Tuesday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian 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, 11
- Ended on
-
Saturday
December 31, 11
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
10s
10–19
- Century
-
1st century
1–100
- Millennium
-
1st millennium
1–1000
- Years ago
-
2,015
2015 years before 2026.
In other calendars
- Hebrew
-
3771 / 3772 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
-
554 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
3 / 4 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
-67 / -68 Saka
Indian national calendar; year starts in March.
Properties
Primality
11 is prime. It has exactly two divisors: 1 and itself.
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√11 = [3; (3, 6)]
Period length 2 — the block in parentheses repeats forever.
Representations
- In words
- eleven
- Ordinal
- 11th
- Roman numeral
- XI
- Binary
- 1011
- Octal
- 13
- Hexadecimal
- 0xB
- Base64
- Cw==
- One's complement
- 244 (8-bit)
- Scientific notation
- 1.1 × 10¹
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒁹
- Egyptian hieroglyphic
- 𓎆𓏺
- Greek (Milesian)
- ιαʹ
- Mayan (base 20)
- 𝋫
- Chinese
- 一十一
- Chinese (financial)
- 壹拾壹
Digit at this position in famous constants
- π — Pi (π)
- Digit 11 = 5
- e — Euler's number (e)
- Digit 11 = 4
- φ — Golden ratio (φ)
- Digit 11 = 7
- √2 — Pythagoras's (√2)
- Digit 11 = 3
- ln 2 — Natural log of 2
- Digit 11 = 5
- γ — Euler-Mascheroni (γ)
- Digit 11 = 9
Also seen as
As an ASCII codepoint, 11 is VT (vertical tab). ASCII control character.
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.11.
- Address
- 0.0.0.11
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.11
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Matches numbered highway designations:
- I-11 — Phoenix, AZ to Las Vegas, NV (expanding).
- US 11 — Rouses Point, NY to New Orleans, LA, paralleling the Appalachians.
As a MIDI note number, 11 is B-1 (15.4 Hz at concert pitch). (below the ~20 Hz floor of human hearing — likely inaudible)
On the periodic table, atomic number 11 is Sodium (Na) — period 3.
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.