20131105, 18:33  #1 
Dec 2011
After milion nines:)
1245_{10} Posts 
Nash weight of base 17
If I wont to test something like 7*17^n1 and 9*17^n1 how to calculate nash weight? Or just to make small sieve and look what sieve is bigger?
Bigger sieve, more weight. 
20131105, 18:40  #2 
"Serge"
Mar 2008
Phi(3,3^1118781+1)/3
3·31·97 Posts 
Nice, easy question. ;)
The weight for either sequence is precisely 0. All values are divisible by 2. 
20131105, 18:44  #3 
Dec 2011
After milion nines:)
3·5·83 Posts 
Ah: read my lips :something like 7*17^n1 and 9*17^n1 :)
14*17^n1 and 16*17^n1 is new examples :)) 
20131105, 18:51  #4 
"Serge"
Mar 2008
Phi(3,3^1118781+1)/3
21475_{8} Posts 

20131106, 08:42  #5 
Mar 2006
Germany
2^{2}×3×233 Posts 
The Nash weight of a number k*2^n+1 (k odd) is defined as the value of n remaining the sieve for 100000 <= n < 110000 and a max factor of 256.
Using srsieve you can try srsieve q n 100000 N 109999 P 256 G "14*17^n1" getting a sieve file with 894 candidates left so Nash weight is here 894. 
20131106, 11:45  #6  
Feb 2003
1,901 Posts 
Quote:
It's the number of remaining n for the interval 100000 <= n < 110000 surviving a Nash sieve with an exponent limit of 256. The Nash sieve doesn't use just the primes up to a given limit (256 in our case) as an ordinary sieve like srsieve does. Instead it sieves against numbers of the type 2^e1 (or more precisely it's factorizations), where the exponent e runs up to the given exponent limit of 256. This makes the implementation of the Nash sieve much more complicated than an ordinary sieve. Regarding the weights this means that a Nash sieve up to e=256 finds more factors than a standard sieve up to p=256. You will notice this difference if you compare the weights obtained by your srsieve procedure to the ones computed either by PSieve or by the Nash weight tool. And regarding pepi37's question: For different bases, like b=17, the Nash sieve as described before doesn't provide a proper sieve base due to the limitation to factors of the type 2^e1. One could define something like a Generalized Nash sieve/weight by using a factor base of b^e1 and the standard exponent limit of 256. Nevertheless, if the aim is just to have some kind of a weight for classifying multiple sequences into low or high weights, the procedure described by kar_bon, e.g. using srsieve up to p=256, might be simpler and faster. Last fiddled with by Thomas11 on 20131106 at 11:47 

20131106, 13:31  #7 
Dec 2011
After milion nines:)
3×5×83 Posts 
So Thomas , karbon example can be used as quick examin of candidate weight?
Where I can download Nash sieve? 
20131106, 14:11  #9  
Feb 2003
1,901 Posts 
Quote:
The original Nash sieve (Psieve) can be found on Chris Nash's homepage: http://pages.prodigy.net/chris_nash/psieve.html However, this link seems to be dead. And the wayback machine doesn't work either at the moment. There is also a Java applet written by Jack Brennen, which can be found here. This one computes the so called Proth weight and is for numbers of the form k*2^n+1, but can also used for the k*2^n1 by entering k as k. The Proth weight is related to the Nash weight by a scaling factor of 1/1751.542, where 1751.542 is just the average Nash weight. My Nash weight tool closely follows Jack Brennen's algorithm and can be found at several places in the forum, e.g. here. Note that neither the Java applet nor my Nash tool can be used for other bases than b=2. 

20131106, 14:30  #10 
Mar 2006
Germany
101011101100_{2} Posts 
Here's psieve3.exe.

20131106, 14:53  #11 
Feb 2003
11101101101_{2} Posts 
Thanks for the file, Karsten!
I did a quick modification to my Nash code and for Pepi37's 14*17^n1 sequence I got the same weight (803) as with PSieve3.exe. I will have to make it a bit more robust before posting a public binary here. But stay tuned... 
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