31
31 is a prime, odd, a calendar year.
31 (thirty-one) is an odd 2-digit number. It is a prime number — divisible only by 1 and itself. It is a Mersenne prime. Written other ways, in Roman numerals it is XXXI and in binary, 11111.
Interestingness
Historical context — 31 AD
Calendar year
AD 31 (XXXI) was a common year starting on Monday of the Julian calendar.
Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 Read the full article on Wikipedia →
Notable events — 31 BC
- Sep 2 Octavian's fleet defeats Antony and Cleopatra at Actium.
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
-
Wednesday
January 1, 31
- Ended on
-
Wednesday
December 31, 31
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
30s
30–39
- Century
-
1st century
1–100
- Millennium
-
1st millennium
1–1000
- Years ago
-
1,995
1995 years before 2026.
In other calendars
- Hebrew
-
3791 / 3792 AM
Rosh Hashanah falls in September/October.
- Chinese
-
Year of the zodiac:Metal zodiac:Rabbit
Sexagenary cycle position 28 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
574 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
23 / 24 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
-47 / -48 Saka
Indian national calendar; year starts in March.
Properties
Primality
31 is prime. It has exactly two divisors: 1 and itself.
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√31 = [5; (1, 1, 3, 5, 3, 1, 1, 10)]
Period length 8 — the block in parentheses repeats forever.
Representations
- In words
- thirty-one
- Ordinal
- 31st
- Roman numeral
- XXXI
- Binary
- 11111
- Octal
- 37
- Hexadecimal
- 0x1F
- Base64
- Hw==
- One's complement
- 224 (8-bit)
- Scientific notation
- 3.1 × 10¹
As an angle
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒌋𒌋𒁹
- Egyptian hieroglyphic
- 𓎆𓎆𓎆𓏺
- Greek (Milesian)
- λαʹ
- Mayan (base 20)
- 𝋡·𝋫
- Chinese
- 三十一
- Chinese (financial)
- 參拾壹
Digit at this position in famous constants
- π — Pi (π)
- Digit 31 = 9
- e — Euler's number (e)
- Digit 31 = 2
- φ — Golden ratio (φ)
- Digit 31 = 5
- √2 — Pythagoras's (√2)
- Digit 31 = 9
- ln 2 — Natural log of 2
- Digit 31 = 8
- γ — Euler-Mascheroni (γ)
- Digit 31 = 2
Also seen as
As an ASCII codepoint, 31 is control character (0x1F). ASCII control character.
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.31.
- Address
- 0.0.0.31
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.31
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Heard as a frequency, 31 Hz is closest to:
- Concert pitch (A4 = 440 Hz): B0 (30.9 Hz, +7¢)
- Scientific pitch (C4 = 256 Hz): B0 (30.2 Hz, +45¢ — about midway to C1)
- Baroque pitch (A4 = 415 Hz): C1 (30.8 Hz, +9¢)
As a MIDI note number, 31 is G1 (49 Hz at concert pitch). (very low — many laptop and phone speakers can't reproduce it)
On the periodic table, atomic number 31 is Gallium (Ga) — period 4.
Related reading
- Mersenne primes — Primes one less than a power of two — the record-holders for the largest known primes for over a century.
- Egyptian hieroglyphic numerals — Seven hieroglyphs for every power of ten, from a single stroke to a million.