Number

51

51 is a composite number, odd, a calendar year.

51 (fifty-one) is an odd 2-digit number. It is a composite number with 4 divisors, and factors as 3 × 17. Written other ways, in Roman numerals it is LI and in binary, 110011.

Arithmetic Number Binary Palindrome Cube-Free Deficient Number Evil Number Motzkin Number Pentagonal Perrin Number Preferred Number Recamán's Sequence Semiprime Squarefree Year

Interestingness

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10 notable facts · grade A

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

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Historical context — 51 AD

Calendar year

AD 51 (LI) was a common year starting on Friday of the Julian calendar.

Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 Read the full article on Wikipedia →

Historical context — 51 BC

Calendar year

Year 51 BC was a year of the pre-Julian Roman calendar.

Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 Read the full article on Wikipedia →

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: Sunday
January 1, 51
Ended on: Sunday
December 31, 51
Friday the 13ths: 2
2 Friday the 13ths this year.
Decade: 50s
50–59
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 1,975
1975 years before 2026.

In other calendars

Hebrew: 3811 / 3812 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Metal zodiac:Pig
Sexagenary cycle position 48 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 594 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 43 / 44 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): -27 / -28 Saka
Indian national calendar; year starts in March.

Properties

Parity: Odd
Digit count: 2
Digit sum: 6
Digit product: 5
Digital root: 6
Palindrome: No
Bit width: 6 bits
Reversed: 15
Recamán's sequence: a(214) = 51
Square (n²): 2,601
Cube (n³): 132,651
Divisor count: 4
σ(n) — sum of divisors: 72
φ(n) — Euler's totient: 32
Sum of prime factors: 20

Primality

Prime factorization: 3 × 17

Nearest primes: 47 (−4) · 53 (+2)

Divisors & multiples

All divisors (4)

1 · 3 · 17 · 51

Aliquot sum (sum of proper divisors): 21

Factor pairs (a × b = 51)

1 × 51

3 × 17

First multiples

51 · 102 (double) · 153 · 204 · 255 · 306 · 357 · 408 · 459 · 510

Sums & aliquot sequence

As consecutive integers: 25 + 26 16 + 17 + 18 6 + 7 + 8 + 9 + 10 + 11

Aliquot sequence: 51 → 21 → 11 → 1 → 0 — terminates at zero

Continued fraction of √n

√51 = [7; (7, 14)]

Period length 2 — the block in parentheses repeats forever.

Representations

In words: fifty-one
Ordinal: 51st
Roman numeral: LI
Binary: 110011
Octal: 63
Hexadecimal: 0x33
Base64: Mw==
One's complement: 204 (8-bit)
Scientific notation: 5.1 × 10¹

In other bases

ternary (3) 1220

quaternary (4) 303

quinary (5) 201

senary (6) 123

septenary (7) 102

nonary (9) 56

undecimal (11) 47

duodecimal (12) 43

tridecimal (13) 3c

tetradecimal (14) 39

pentadecimal (15) 36

Palindromic in base 2, base 4

As an angle

51°

51° ≈ 0.89 rad

Compass bearing: NE (northeast)

Why does a circle have 360 degrees? →

Historical numeral systems

Babylonian (base 60): 𒌋𒌋𒌋𒌋𒌋𒁹
Egyptian hieroglyphic: 𓎆𓎆𓎆𓎆𓎆𓏺
Greek (Milesian): ναʹ
Mayan (base 20): 𝋢·𝋫
Chinese: 五十一
Chinese (financial): 伍拾壹

In other modern scripts

Eastern Arabic ٥١ Devanagari ५१ Bengali ৫১ Tamil ௫௧ Thai ๕๑ Tibetan ༥༡ Khmer ៥១ Lao ໕໑ Burmese ၅၁

Digit at this position in famous constants

π — Pi (π): Digit 51 = 0
e — Euler's number (e): Digit 51 = 5
φ — Golden ratio (φ): Digit 51 = 6
√2 — Pythagoras's (√2): Digit 51 = 4
ln 2 — Natural log of 2: Digit 51 = 5
γ — Euler-Mascheroni (γ): Digit 51 = 2

Also seen as

ASCII character

As an ASCII codepoint, 51 is 3. Printable ASCII character 3.

Hex color

#000033

RGB(0, 0, 51)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.51.

Address: 0.0.0.51
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.51

Unspecified address (0.0.0.0/8) — "this network" placeholder.

Calculator-display word

Type 51 on a seven-segment calculator, flip it 180°, and the display reads:

A staple of calculator humor since pocket calculators put digits in front of bored students.

Musical pitch

Heard as a frequency, 51 Hz is closest to:

Concert pitch (A4 = 440 Hz): G♯1 (51.9 Hz, -31¢)
Scientific pitch (C4 = 256 Hz): G♯1 (50.8 Hz, +7¢)
Baroque pitch (A4 = 415 Hz): A1 (51.9 Hz, -29¢)

(very low — many laptop and phone speakers can't reproduce it)

As a MIDI note number, 51 is D♯3 (155.6 Hz at concert pitch).

Atomic number

On the periodic table, atomic number 51 is Antimony (Sb) — period 5.

Antimony on Wikipedia ↗