19 (number)
19 (nineteen) is the natural number following 18 and preceding 20.
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.
The decimal expansion of 1/19 has a period of maximum length: 18 digits.
Nineteen is a Heegner number.
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
