Number

15

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

15 (fifteen) is an odd 2-digit number. It is a composite number with 4 divisors, and factors as 3 × 5. It is the fifth triangular number. Written other ways, in Roman numerals it is XV and in binary, 1111.

Arithmetic Number Bell Number Binary Palindrome Cake Number Cube-Free Deficient Number Evil Number Hexagonal Magic Constant Partition Number Pentatope Number Preferred Number Recamán's Sequence Semiprime Squarefree Tetranacci Number Triangular Year Zuckerman Number

Interestingness

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16 notable facts · grade A

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Historical context — 15 AD

Calendar year

AD 15 (XV) was a common year starting on Tuesday of the Julian calendar.

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

Historical context — 15 BC

Calendar year

Year 15 BC was either a common year starting on Tuesday, Wednesday or Thursday or a leap year starting on Wednesday of the Julian calendar and a common year starting on Monday of the Proleptic Julian 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: 53
Long year: contains 53 ISO weeks.
Started on: Thursday
January 1, 15
Ended on: Thursday
December 31, 15
Friday the 13ths: 3
3 Friday the 13ths this year.
Decade: 10s
10–19
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 2,011
2011 years before 2026.

In other calendars

Hebrew: 3775 / 3776 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Wood zodiac:Pig
Sexagenary cycle position 12 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 558 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 7 / 8 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): -63 / -64 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: 4 bits
Reversed: 51
Recamán's sequence: a(29) = 15
Square (n²): 225
Cube (n³): 3,375
Divisor count: 4
σ(n) — sum of divisors: 24
φ(n) — Euler's totient: 8
Sum of prime factors: 8

Primality

Prime factorization: 3 × 5

Nearest primes: 13 (−2) · 17 (+2)

Divisors & multiples

All divisors (4)

1 · 3 · 5 · 15

Aliquot sum (sum of proper divisors): 9

Factor pairs (a × b = 15)

1 × 15

3 × 5

First multiples

15 · 30 (double) · 45 · 60 · 75 · 90 · 105 · 120 · 135 · 150

Sums & aliquot sequence

As consecutive integers: 7 + 8 4 + 5 + 6 1 + 2 + 3 + 4 + 5

Aliquot sequence: 15 → 9 → 4 → 3 → 1 → 0 — terminates at zero

Continued fraction of √n

√15 = [3; (1, 6)]

Period length 2 — the block in parentheses repeats forever.

Representations

In words: fifteen
Ordinal: 15th
Roman numeral: XV
Binary: 1111
Octal: 17
Hexadecimal: 0xF
Base64: Dw==
One's complement: 240 (8-bit)
Scientific notation: 1.5 × 10¹

In other bases

ternary (3) 120

quaternary (4) 33

quinary (5) 30

senary (6) 23

septenary (7) 21

nonary (9) 16

undecimal (11) 14

duodecimal (12) 13

tridecimal (13) 12

tetradecimal (14) 11

pentadecimal (15) 10

Palindromic in base 2

Historical numeral systems

Babylonian (base 60): 𒌋𒁹𒁹𒁹𒁹𒁹
Egyptian hieroglyphic: 𓎆𓏺𓏺𓏺𓏺𓏺
Greek (Milesian): ιεʹ
Mayan (base 20): 𝋯
Chinese: 一十五
Chinese (financial): 壹拾伍

In other modern scripts

Eastern Arabic ١٥ Devanagari १५ Bengali ১৫ Tamil ௧௫ Thai ๑๕ Tibetan ༡༥ Khmer ១៥ Lao ໑໕ Burmese ၁၅

Digit at this position in famous constants

π — Pi (π): Digit 15 = 9
e — Euler's number (e): Digit 15 = 4
φ — Golden ratio (φ): Digit 15 = 9
√2 — Pythagoras's (√2): Digit 15 = 9
ln 2 — Natural log of 2: Digit 15 = 4
γ — Euler-Mascheroni (γ): Digit 15 = 3

Also seen as

ASCII character

As an ASCII codepoint, 15 is control character (0x0F). ASCII control character.

Hex color

#00000F

RGB(0, 0, 15)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.15.

Address: 0.0.0.15
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.15

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

US numbered highway

Matches numbered highway designation:

I-15 — San Diego, CA to Sweet Grass, MT (Canadian border).

Interstate 15 on Wikipedia ↗

Musical pitch

As a MIDI note number, 15 is D♯0 (19.4 Hz at concert pitch). (below the ~20 Hz floor of human hearing — likely inaudible)

Atomic number

On the periodic table, atomic number 15 is Phosphorus (P) — period 3.

Phosphorus on Wikipedia ↗