19

The Numbers were a constant throughout the series. Fans want the Numbers to mean more than the explanation given by TPTB.

In typical in-series episode clue hunting, scratch math of the Numbers:

If we look at the "progression" of the Numbers, we find an interesting pattern. The number "19" is produced by the first four number sets and by the final number set.

How does the Number 19 apply to LOST?

In the lighthouse, candidate 19 was Nguyen, and that name was crossed out. Lostpedia references the trivia of brutal Vietnamese leader Ho Chi Mihn was born Nguyen.

In the chart of elements, Number 19 is for potassium. It's symbol is K. 

K is the eleventh letter of the alphabet.It's abbreviation denotes things like 
• kelvin(s).
• Computing kilobyte(s).
• kilometer(s).
• kindergarten.
• king (used esp. in describing play in card games and recording moves in chess):
• knit (as an instruction in knitting patterns):
• (also k ) informal thousand
• Baseball strikeout.
• karat.
• a constant in a formula or equation.
• Chemistry Boltzmann's constant,  the ratio of the gas constant to Avogadro's number, equal to 1.381 × 10−23 joules per kelvin. (Symbol: k ).

It is interesting to note so many things pop up from K. Kelvin was the character in the Hatch who lied to Desmond to keep him imputing the Numbers into the computer. He was also a military officer Many people think the characters interactions with each other was like kindergarten children. Everyone wanted to be king of the island. The turning point for Jack, in captivity, learned  the Red Sox won the World Series, reversing a fan curse. We were also told about "constants" by Daniel as the means of surviving the island's unique properties, including time travel.

Potassium is a metal and is the seventh most abundant and makes up about 1.5 % by weight of the earth's crust. Potassium is an essential constituent for plant growth and it is found in most soils. It is also a vital element in the human diet.

Potassium is never found free in nature, but is obtained by electrolysis of the chloride or hydroxide. It is one of the most reactive and electropositive of metals and, apart from lithium, it is the least dense known metal. It is soft and easily cut with a knife. It is silvery in appearance immediately after a fresh surface is exposed.

So the Number 19 is a number derived from LOST's series of numbers. Is it a clue that the key to understanding LOST can be learned through Kelvin? Probably not. He was a minor character who conveniently shows up on two different characters' background stories. The themes attached to him include military, duty, manipulation, lies, and wanting to escape. Potassium is never found "free" in nature; it must be manipulated to be obtained. Again, the clue could be that the characters had to be manipulated by the island in order to escape the traps and lies of their human existence.