20170120, 08:51  #1 
Sep 2002
Database er0rr
11×13×29 Posts 
Another way to PRP test Mersenne numbers
Not as efficient as LL:
Code:
yaMersennePRP(p)=local(n=2^p1,a=Mod(4,n),b=a+1);for(k=2,p,a=2*a*b;b=a+1);print(p" "a==4) 
20170120, 12:41  #2  
"Forget I exist"
Jul 2009
Dumbassville
2^{6}·131 Posts 
Quote:
1) getting rid of b is easy as at every step using it you are calculating 2*a^2+2*a I had a few more at first but I tested them and they didn't work to replicate, they went down the wrong division of everything by 2 path I think. 

20170120, 23:25  #3 
"Rashid Naimi"
Oct 2015
Remote to Here/There
2·1,123 Posts 
Isn't LL a primality test rather than a PRP test?
Is this algo also a deterministic primality test? 
20170121, 00:28  #4 
Sep 2002
Database er0rr
1033_{16} Posts 

20170121, 00:48  #5 
"Rashid Naimi"
Oct 2015
Remote to Here/There
2·1,123 Posts 
So I assume you know of values for p which return true but are in fact yield composite Mersennes. Correct?
Last fiddled with by a1call on 20170121 at 00:48 
20170121, 01:27  #6 
Sep 2002
Database er0rr
11·13·29 Posts 

20170121, 01:38  #7 
"Rashid Naimi"
Oct 2015
Remote to Here/There
2246_{10} Posts 
Well, that makes it more interesting than just another PRP test.
Are there any values that you know of, that yield false but are in fact prime? That shouldn't be too difficult to check given the limited number of known MPs. 
20170121, 05:31  #8  
Aug 2006
3×1,993 Posts 
Quote:
Code:
#include <pari/pari.h> /* For use in gp: GP;install("testMersenne","lU","testMersenne","./filename.gp.so"); */ long testMersenne(ulong p) { if (p < 4) return p > 1; pari_sp av = avma; GEN n = subiu(int2u(p), 1); pari_sp btop = avma; GEN a = utoipos(4); ulong k; long ret; for (k = 2; k <= p; ++k) { a = remii(shifti(addii(sqri(a), a), 1), n); if (gc_needed(btop, 1)) a = gerepileuptoint(btop, a); } ret = absequaliu(a, 4); avma = av; return ret; } 

20170121, 11:53  #9 
Sep 2002
Database er0rr
4147_{10} Posts 

20170121, 12:00  #10 
Jun 2003
3×1,789 Posts 
This test will work with other seeds of the form 2*a*(a+1) like 4,12,24,40, etc..
Interestingly some of them fail for p=11, declaring it to be prime (for eg: 60, 760) 
20170122, 09:38  #11 
"Rashid Naimi"
Oct 2015
Remote to Here/There
2·1,123 Posts 

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