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 2022-08-12, 09:07 #45 LaurV Romulan Interpreter     "name field" Jun 2011 Thailand 17·19·31 Posts Sorry, busy here around, I just looked for this thread, which you should read carefully, especially from post #32 onward, till the end.
2022-08-12, 22:37   #46
User140242

Jul 2022

2610 Posts

Quote:
 Originally Posted by LaurV Sorry, busy here around, I just looked for this thread, which you should read carefully, especially from post #32 onward, till the end.

In addition you can calculate the possible remainders and automatically you can find the possible factors:

Code:
tf_c(p,fromk=0,tok=10000)=
{
PrimesBase=[2,3,5,7,11];
n_PB=5;
PrimesBaseProd=2;
for(i=1,n_PB,PrimesBaseProd*=PrimesBase[i];);
bW=2*p*PrimesBaseProd;
RW=List();
listput(RW,1);
for(i=1,PrimesBaseProd-1,for(j=1,n_PB,;if(Mod(1+2*p*i,PrimesBase[j])^1==0,break;);if ((j==n_PB)&&(Mod(1+2*p*i,PrimesBase[j])^1!=0)&&(Mod(1+2*p*i,8)^1==1||Mod(1+2*p*i,8)^1==7),listput(RW,1+2*p*i););););
nR=length(RW);
for(k=fromk, tok, for(i=1,nR,q=RW[i]+bW*k; if(Mod(2,q)^p==1, if(q>1,print("M"p" has a factor: "q); break;));));
}

Last fiddled with by User140242 on 2022-08-12 at 22:38

 2022-08-13, 03:56 #47 LaurV Romulan Interpreter     "name field" Jun 2011 Thailand 234358 Posts Why would you do that? It will take a lot of time to build that list, and it will find a lot of unneeded composite factors which you need to factor later to find the real prime factors.
2022-08-13, 06:24   #48
User140242

Jul 2022

2·13 Posts

Quote:
 Originally Posted by LaurV Why would you do that? It will take a lot of time to build that list, and it will find a lot of unneeded composite factors which you need to factor later to find the real prime factors.

You can use n_PB as a parameter and if you check the number of remainders that nR uses they are 2 if n_PB = 1, 4 if n_PB = 2, 16 if n_PB = 3, 96 if n_PB = 4 and 960 if n_PB = 5.

That is, it is the same job as creating the table but you can perform an automatic operation as needed.

Of course with this method it checks (tok-fromk)*nR elements instead with forstep it checks (tok-fromk)/nR elements but the factors are the same only the size of the interval changes.

Finally, multiples of small factors can be easily eliminated for each row.

Here you find an example with 2 classes and to speed up we use two vectors for the two classes:
https://gist.github.com/user140242/0...d2738788bb1cae

Last fiddled with by User140242 on 2022-08-13 at 06:45

2022-08-13, 09:57   #49
User140242

Jul 2022

2×13 Posts

Quote:
 Originally Posted by User140242 Finally, multiples of small factors can be easily eliminated for each row.

To extend the example for classes greater than 2 for each class we must use this:

It is possible to eliminate the multiples of the small factors p_j>PrimesBase[n_PB] with p_j<bW and p_j!=p and p_j!=RW[n]
for each RW[i] take a vector FACTOR of size dim_step+1 with dim_step=(tok-fromk) initially set to True

all multiples of p_j can be set to False in this way:

Code:
r_t1=Euclidean_Diophantine(bW,p_j); //function return y in  Diophantine equation  bW * x + p_j * y  = 1
r_t=(r_t1*RW[i])%bW;
if (r_t>0)
r_t-=bW;
C2=(-bW+p_j)*r_t/bW;
C1=r_t-bW+p_j;
for (m =bW+C1+C2; m<=dim_step && m>=0; m += p_j)
FACTOR[m] = false;
and then use it in this way to check for possible factors:

Code:
for(k=0;k<=dim_step;k++)
if (FACTOR[k])
{
D=RW[i]+bW*k;
....
}

Last fiddled with by User140242 on 2022-08-13 at 10:15

2022-08-13, 10:15   #50
Dobri

"Καλός"
May 2018

73 Posts

Quote:
 Originally Posted by Dr Sardonicus ... For p == 1 (mod 4), this automatically rules out q = 2*k*p + 1 for k = 12*m + 1, 12*m + 5, or 12*m + 9 since 2*k*p + 1 == 3 (mod 8) for such k.
Thanks, this nicely eliminates the cases for k = 12m + 1, 12m + 5, and 12m + 9 indeed.
Quote:
 Originally Posted by Dobri ... 2(12m+8)p + 1, ... 2(12m+11)p + 1, m = 0, 1, 2,..., cannot be factors of M1277.
However, the cases for k = 12m + 8 and 12m + 11 still need a theoretical interpretation.
I do not have a counterexample with p mod 6 = 5 that would result in a prime q = 2kp + 1 for k = 12m + 8 or 12m + 11 that divides 2p - 1.
Currently, I am checking the known prime factors in the GIMPS database for a counterexample.
Such a counterexample with p mod 6 = 5 could be rare or does not exist.

2022-08-13, 11:10   #51
charybdis

Apr 2020

22×3×67 Posts

Quote:
 Originally Posted by Dobri However, the cases for k = 12m + 8 and 12m + 11 still need a theoretical interpretation. I do not have a counterexample with p mod 6 = 5 that would result in a prime q = 2kp + 1 for k = 12m + 8 or 12m + 11 that divides 2p - 1. Currently, I am checking the known prime factors in the GIMPS database for a counterexample. Such a counterexample with p mod 6 = 5 could be rare or does not exist.
These have the simplest explanation of all. If p and k are both 2 mod 3 then 2kp+1 is divisible by 3 and so can't be prime.

2022-08-14, 07:59   #52
Dobri

"Καλός"
May 2018

73 Posts

Quote:
 Originally Posted by charybdis These have the simplest explanation of all. If p and k are both 2 mod 3 then 2kp+1 is divisible by 3 and so can't be prime.
Thanks, this flawlessly eliminates the cases for k = 12m + 8 and 12m + 11 indeed.
Also, divisibility by 3 is observed for k = 12m + 5 since 2(12m + 5)(6n + 5) + 1 gives the coefficient 2×5×5 + 1 = 51 which is divisible by 3.
Despite the equivalence between 2 mod 3 and 5 mod 6 (and between 1 mod 3 and 1 mod 6) for prime number considerations, I still use 'mod 6' for uniformity with a few other threads.

What I am trying to evaluate is the additive dependence between the values of k for several factors for a given Mersenne number.
On the basis of quite limited observations for k up to 11 (see posts #29 and #30), the values of k for another factor are obtained as follows:
- the sum of a prime number (or 1) and 2n such as 5 + 22 = 32 and 1 + 23 = 32;
- the sum of 1 and a prime number giving 2n such as 1 + 3 = 22 and 1 + 7 = 23;
- the sum of two prime numbers giving 2n such as 3 + 5 = 23; or
- the sum of 2n and a prime number giving a prime number such as 22 + 3 = 7 and 23 + 3 = 11.

The exhaustive computations for k = 12 are ongoing. The custom source code was modified to collect data not only for p < 109 but also for p < 230.

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