# 73 -> 2n+1 ever prime?

Robert G. Wilson v rgwv at southwind.net
Tue Dec 21 17:18:06 CET 1999

```Et al,

Using Mathematica 4,
a=73;a=2*a+1;n=1;While[!PrimeQ[a],a=2*a+1;n++];Print[n,"   ",a], I found at
the 2552rd iteration the Probable Prime listed below.

125250842032596022141763451178279918575730634371510796501896566892520416173991611861897687317443664819437820214560609681743335031976337579413232699338320014217732225003163760036417965916387747831867749318699104524437655151695087826472783577318243917295323190691889073505394189591684259401693565321954263531958425718352075521212919447463091987941305734624780007152400868604948878094276638123436651683349651892026768245860789398297612527549211852109219078820059778193464322428143746090914137892405985983359244639484199470043684570225177660349559179987031165034324694388497208369119597566358566756071628978550352418235553897768571561351251352502155056787443177087759615376430034900988921205572639317118528079725593399200244440233458975807425711011346463660588817113315016703

or approximately 1.2525 E770  in about 10 minutes on a PII  laptop.

Robert G. 'Bob' Wilson v

"Christian G.Bower" wrote:

> I was looking at the EIS and found sequence A051914 where Neil was
> seeking clarification. So I worked out what its meaning and that led
> to the following computation.
>
> Take 73 then iterate f(n)=2n+1 on the value to get
>
> 147 295 591 1183 ...
>
> until I get a value that's prime.
>
> So far I'm up to
>
> 6759445072655425996692042351312808965908688011263
>
> without finding one.
>
> Perhaps someone with a smart prime testing algorithm can do the search
> or someone with some theoretical knowledge can tell me the search is
> in vain.
>
> Christian
>
> ____________________________________________________________________