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← 18 19 20 →
Cardinalnineteen
Ordinal19th
(nineteenth)
Numeral systemnonadecimal
Factorizationprime
Prime8th
Divisors1, 19
Greek numeralΙΘ´
Roman numeralXIX
Binary100112
Ternary2013
Octal238
Duodecimal1712
Hexadecimal1316

19 (nineteen) is the natural number following 18 and preceding 20. It is a prime number.

Mathematics

19 is a centered triangular number

19 is the 8th prime number, the seventh Mersenne prime exponent,[1] and the second base-10 repunit prime exponent.[2] 19 is the maximum number of fourth powers needed to sum up to any natural number, and in the context of Waring's problem, 19 is the fourth value of g(k). In addition, 19 is a Heegner number[3] and a centered hexagonal number.[4] 19 is also a centered triangular number,[5] being the smallest such number that is also prime. 19 is also the second Keith number and also the first Keith prime. Moreover, there exists only one nontrivial normal magic hexagon: the one composed of 19 cells.

Science

Religion

Islam

Baháʼí faith

In the Bábí and Baháʼí Faiths, a group of 19 is called a Váhid, a Unity (Arabic: واحد wāhid, "one"). The numerical value of this word in the Abjad numeral system is 19.

Celtic paganism

19 is a sacred number of the goddess Brigid because it is said to represent the 19-year cycle of the Great Celtic Year and the amount of time it takes the Moon to coincide with the winter solstice.[6]

Music

Literature

Games

A 19x19 Go board
A 19x19 Go board

Age 19

In sports

Other fields

References

  1. ^ "Sloane's A000043 : Mersenne exponents". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
  2. ^ Guy, Richard; Unsolved Problems in Number Theory, p. 7 ISBN 1475717385
  3. ^ "Sloane's A003173 : Heegner numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
  4. ^ "Sloane's A003215 : Hex (or centered hexagonal) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
  5. ^ "Sloane's A125602 : Centered triangular numbers that are prime". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
  6. ^ Brigid: Triple Goddess of the Flame (Health, Hearth, & Forge)
  7. ^ Roush, Gary (2008-06-02). "Statistics about the Vietnam War". Vietnam Helicopter Flight Crew Network. Archived from the original on 2009-12-07. Retrieved 2009-12-06. Assuming KIAs accurately represented age groups serving in Vietnam, the average age of an infantryman (MOS 11B) serving in Vietnam to be 19 years old is a myth, it is actually 22. None of the enlisted grades have an average age of less than 20.