October 8th, 2015, 08:05 AM  #1 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41  New primality test
This primality test is different from Fermat test : n=561 which is a Carmichael number fail this test. So I still do not know if some numbers fail this test. Are they finite or no? Thank you for listing the first numbers failing this test. Thank you. n positive odd integer > 3 Let us define a function S(k) S(n)=2^n S(n1)=S(n)(2^n1) S(n2)=S(n1)+(2^n2) S(n3)=S(n2)(2^n3) .... S(0)=S(1)+ or  (2^0) We start with the sign + and we alternate +++.... Example n=5 S(5)=2^5=32 S(4)=2^52^4=3216=16 S(3)=16+2^3=24 S(2)=242^2=20 S(1)=20+2=22 S(0)=22(2^0)=221=21 Here is the algorithm : 1. Compute S(2) 2. Compute gcd(n,S(2) If 1<gcd(n,S(2))<n then n is composite print "n is composite" otherwise next step 3. Compute S(2) mod n If S(2) mod n = 0 then n is prime print "n is prime" otherwise n is composite 4. End 
October 8th, 2015, 09:30 AM  #2 
Member Joined: Oct 2013 Posts: 60 Thanks: 6 
Here's your Code Code: ismobel(n)={ \\returns 1, if n is a (pseudo)prime, otherwise 0. my(s2=sum(x=2,n,2^x*(1)^(x1)),n1=gcd(n,s2)); if(n1>1&&n1<n,return(0),s2%n==0,return(1)) } Code: for(n=4,10^4,if(isprime(n)<>ismobel(n),print(n))); 4 341 946 1105 1387 1729 2047 2465 2701 2821 3277 4033 4369 4681 5461 6601 7957 8321 8911 time = 20,661 ms. 
October 8th, 2015, 09:35 AM  #3 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41 
Thank you. It seems that there is an infinite number of those failing the test. So I see no need to find a close formula for computing S(2) without going through all the values from n to 2. Gcd(n,S(2)) seems unuseful then. Thank you very much. 
October 8th, 2015, 09:37 AM  #4 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41 
Only odd numbers so 4 and 946 does noy count.

October 8th, 2015, 09:48 AM  #5 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 
Interesting. This seems to be very similar to a base2 Fermat test combined with a divisibility by 3 test.

October 8th, 2015, 09:57 AM  #6 
Member Joined: Oct 2013 Posts: 60 Thanks: 6 
I misssed to filter the Test for odd numbers! 946 seems to be the only even pseudoprime for n>4. And 561 is missing as gcd(s2,561)=187 
October 8th, 2015, 10:18 AM  #7  
Member Joined: Oct 2013 Posts: 60 Thanks: 6  Quote:
Code: forstep(n=5,3*10^4,2,if(isprime(n)<>ismobel(n),print1(n,", "))); 341, 1105, 1387, 1729, 2047, 2465, 2701, 2821, 3277, 4033, 4369, 4681, 5461, 6601, 7957, 8321, 8911, 10261, 10585, 11305, 13741, 13747, 13981, 14491, 15709, 15841, 16705, 18721, 19951, 23377, 29341 time = 3min, 11,981 ms.  
October 8th, 2015, 10:22 AM  #8 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41 
There is still a room for improving the test. First : finding close formula for computing S(2) Second : analyzing the numbers failing the test. We could then reach almost 100% success I`m still thinking on how to do ti. If you have any idea let us improve the test. 
October 8th, 2015, 10:22 AM  #9 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  
October 8th, 2015, 10:26 AM  #10  
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Quote:
https://oeis.org/A066488 which shows that Robert G. Wilson v had considered this at least 13 years ago. On the plus side, this gives a really quick version of the test (see the 2011 comment or the 2013 program). On the minus, this suggests that strengthening it will be hard.  

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