20101109, 01:21  #1 
Jul 2007
17_{10} Posts 
MillerRabin Strong Probable Prime Test (SPRP)
From Chris Caldwell's Prime pages (1) (2) , there's a definition of Strong Probable Primes. It matches Wikipedia and a few other googled sources.
But this test doesn't seem to work. Let's test n=7 with the SPRP base a=35. n1=6, so d=3, s=1. 35^3 mod 7 = 0. This isn't 1, so it fails the first test. (35^3)^2 mod 7 =0. This isn't 1, so it fails the second test. Therefore 7 is not an 35SPRP. So 7 is not prime. What's the flaw in the logic above? My bad math? Is it just a bad definition which should add some restriction that composite base values a are not allowed? Or another unlisted requirement that a mod n cannot be 0? 
20101109, 01:58  #2  
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
17×251 Posts 
35 is a multiple of 7. So of course 35^3 = 0 mod 7, along with any powers of 35^3. Maybe there's a condition that isn't being made clear, that a and p must be coprime, or at least that a can't be a multiple of p.
Composite bases are allowed, though: Quote:
Last fiddled with by MiniGeek on 20101109 at 02:20 

20101109, 03:05  #3 
Aug 2006
3·1,993 Posts 
Generally, you want to choose bases between 2 and p2, inclusive. You won't run into trouble in this range.

20101109, 05:44  #4 
Jul 2007
11_{16} Posts 
Thanks for verifying my math!
So the question is actually whether the definition of an SPRP is wrong and needs a special caveat for these cases, or whether it's OK for primes to fail the SPRP test. Should we just say that "n is an aSPRP" is undefined if n < a+2? Or just say that all values n <=a are an aSPRP. Both definitions preserve the useful fact that if n is not an aSPRP, it is composite. 
20101109, 12:36  #5  
Nov 2003
2^{2}·5·373 Posts 
Quote:
specify (a,p) = 1. 

20101109, 13:13  #6  
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
17·251 Posts 
Quote:
Last fiddled with by MiniGeek on 20101109 at 13:27 

20101109, 13:35  #7  
Nov 2003
2^{2}·5·373 Posts 
Quote:
Standard number theoretic usage. 

20101109, 14:41  #8 
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
10253_{8} Posts 
That doesn't help me. Will you please tell me what it is? Or perhaps, is there a resource that would help me understand standard number theoretic usages when it's not spelled out?
If not, will someone else tell me what he means? Last fiddled with by MiniGeek on 20101109 at 14:41 
20101109, 14:48  #9  
Nov 2003
1110100100100_{2} Posts 
Quote:
Numbers. Or any other text on elementary number theory. I can recommend several others if Hardy & Wright is not to your liking. As to why I won't just *give* the answer: Give a man a fish and he eats for a day. Teach a man to fish and he eats for a lifetime. 

20101109, 15:11  #10 
Aug 2006
3×1,993 Posts 

20101109, 15:26  #11  
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
17×251 Posts 
Quote:
Thanks. 

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