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 A258868 a(n) is the smallest integer >= a(n-1) such that prime(n)*2^a(n)-1 is a prime number. 1
 1, 1, 2, 5, 26, 287, 356, 395, 544, 11008, 21957, 32125, 42450, 50867, 55408, 206970, 358276, 384287, 403461, 735802, 783831, 969795, 1192950, 1383108 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS K. Bonath, Riesel and Proth Prime Database (2015) EXAMPLE 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. 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 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 *) PROG (PARI) first(n)=my(t, p); vector(n, i, p=prime(i); while(!ispseudoprime(p<

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Last modified May 31 16:29 EDT 2020. Contains 334748 sequences. (Running on oeis4.)