# 19 (number)

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

 Template:Numbers 10s Cardinal nineteen Ordinal nineteenth Factorization prime Divisors 1, 19 Roman numeral XIX Binary 10011 Hexadecimal 13
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## In mathematics

Nineteen is the 8th smallest prime number. The sequence continues 23, 29, 31, 37... 19 is the seventh Mersenne prime exponent.

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 1, 9, 10, 19...

With respect to divisibility, nineteen has a very simple test: 19 divides 100*a+b if and only if it divides a+4*b.

Nineteen is a centered triangular number, centered hexagonal number, and an octahedral number.

Every positive integer is the sum of at most nineteen fourth powers (see Waring's problem).

The decimal expansion of 1/19 has a period of maximum length: 18 digits.

Nineteen is a Heegner number.

## In science

In astronomy,

Messier object M19, a magnitude 8.5 globular cluster in the constellation Ophiuchus
The New General Catalogue object (http://www.ngcic.org/) NGC 19, a barred spiral galaxy in the constellation Andromeda
The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -2333 June 15 and ended on -1035 August 1. The duration of Saros series 19 was 1298.1 years, and it contained 73 solar eclipses.
The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -2031 March 24 and ended on -733 May 11. The duration of Saros series 19 was 1298.1 years, and it contained 73 lunar eclipses.

## In music

"19" is a 1985 hit song by Paul Hardcastle, with a strong anti-war message, based on 19 being the average age of soldiers in the Vietnam war.

Other songs with the number in their titles include "Nineteen" by the Old 97's and "Hey, Nineteen" by Steely Dan.

## In other fields

Nineteen is

In Stephen King's Dark Tower series, 19 is a number of magical significance, as the characters see 19 practically wherever they travel. In Book Seven, it is revealed that the 19s that the ka-tet has been seeing eludes to June 19, 1999, where the Crimson King is conspiring to kill Stephen King so the Dark Tower will fall. Roland, Jake, and Oy travel to 1999 to avert King's death, but since they altered King's destiny, they pay a devastating price. Jake Chambers sacrifices himself to save King's life so the Tower will stand.

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