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As we know, half-life is the time in which half of radioactive isotops will decay. For example, for 232U (uranium 282) it takes 68.9 years to loose half of it's mass. That's the232U's half-life.
Now let's think about Half-Life series. Valve was found in 1996, HL was released in late 1998. Half-Life2 was released in late 2004. Shouldn't we think mathematically?
If half-life for an element is 2 years, then half-life2 for that element is 4 years (half-life*half-life=2*2).
If Valve was found in 1996 and Half-Life (which is equal to Half-Life1) was released in late 1998, than it took at least 2 years to develop Half-Life.
6 years after HL1 release Half-Life2 was released, so, let's find a quad root of Half-Life2. SQRT(6)=2,44... Did it take 2.44 years to develop Half-Life1? It's pretty close to the real data. And what happens if we literally count Half-Life3?
2,44^3=2,44*2,44*2,44=14,5 years. Let's take HL2 release date (late 2004 = 2004.8) and add 14,5 years which are equal to Half-Life3 - we get 2019. Now you know the release date :)
P.S.: To get more accurate results, it's better to count days instead of years, which I'm going to do later.
P.P.S.: Please, if you re-post this info, don't forget to give link to this blog! Thanks!