FFt length

[mack]

what is the fft length and how does it effect speed?

which test actually only look for numbers that aren't prime and how does it help?

when I see something like 2^68 what does it mean? I know its an exponent but i happen to see it a lot.

[Uncwilly]

Originally Posted by mack

what is the fft length and how does it effect speed?

FFT stands for Fast Fourier Transform, the bigger the number that you are testing, the larger the FFT length. It has to be big enough to handle the number. The larger the FFT, the harder the computer has to work. So, the larger the FFT, the slower the test runs, but there is little that you can do about it (only near the edges)

which test actually only look for numbers that aren't prime and how does it help?

The Trial Factoring test will not find a prime, it only finds factors of numbers that aren't prime. This helps, because it is faster than the much longer Lucas-Lehmer test (this is the one that proves that a number is prime.) By eliminating many numbers (60% or more) using the TF test, this saves many, many tests. P-1 also can find factors.

when I see something like 2^68 what does it mean? I know its an exponent but i happen to see it a lot.

It means that the program is looking for factors of the number that you are testing, that are about 2^68 in size (147,000,000,000,000,000,000 -> 295,000,000,000,000,000,000 = 295 quintillion).