Thank you, John. Exactly what I was hoping for: A concise reply. While I have no appreciation of its significance (nor do I need to), I understand the explanation.
RD  I made it perfectly clear that I do not understand how the LL test works and that I was demonstrating my ignorance.
Regarding "starting at each end ... and meeting somewhere in the middle", I know exactly what I mean and so would you if you would "apply a little common sense."
Just for your benefit, RD, if I wanted to test 1234567 for primality I could, for example, use a simple arithmetic test to discover if any integer between 2 and SQRT(1234567) was an exact divisor of 1234567. If I chose to I could increase the speed of my test by running it twice, one test starting at 2 and working upwards, the other starting at SQRT(1234567) and working downwards. "somewhere in the middle" the two would converge.
Now I am NOT suggesting that the LL test works like this. In fact I was at pains to make clear that I have no understanding of how it works.
"We have an initial starting value: L_0 = 4. We are trying to compute a final value: L_n. How do you propose we "start at each end" if we don't even know what the ending value is???" means nothing to me.
"FFT" has no meaning to me beyond the words the letters represent.
Congratulations, RD, for once again pointlessly attacking a selfconfessed "Ignorant GIMPSer".
