A258868 a(n) is the smallest integer >= a(n-1) such that prime(n)*2^a(n)-1 is a prime number.
1, 1, 2, 5, 26, 287, 356, 395, 544, 11008, 21957, 32125, 42450, 50867, 55408, 206970, 358276, 384287, 403461, 735802, 783831, 969795, 1192950, 1383108
Offset: 1
Keywords
Examples
2*2^1-1=3 prime so a(1)=1. 3*2^1-1=5 prime so a(2)=1. 5*2^1-1=9 composite, 5*2^2-1=19 prime so a(3)=2.
Links
- K. Bonath, Riesel and Proth Prime Database (2015)
Crossrefs
Cf. A128979.
Programs
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Maple
A258868 := proc(n) option remember; if n = 0 then 0; else for a from procname(n-1) do ithprime(n)*2^a-1 ; if isprime(%) then return a; fi ; end do: end if; end proc: # R. J. Mathar, Sep 23 2016
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Mathematica
lst={1};Do[x=Last[lst];Label[begin]; If[PrimeQ[Prime[n]*2^x-1],AppendTo[lst,x],x=x+1;Goto[begin]],{n,2,9}];lst (* Ivan N. Ianakiev, Jun 19 2015 *) -
PARI
first(n)=my(t,p); vector(n,i, p=prime(i); while(!ispseudoprime(p<