A340214 - OEIS

%I #11 Jan 01 2021 12:24:42

%S 3,55,4,30,37,11,25,363,31,13,312,10,78,16,67,53,243,26,35,74,39,235,

%T 709,102,224,1192,33,207,103,94,192,940,19,833,633,99,64,680,261,434,

%U 3994,105,743,988,68,367,168,440,177,1874,36,567,3810,468,250,1006,52,226,1514,38,1313,3277,160,737

%N a(n) is the least m such that A340212(m) = 2*n.

%C 2*n*prime(a(n))+prime(a(n)-1) and 2*n*prime(a(n))+prime(a(n)+1) are prime.

%H Robert Israel, <a href="/A340214/b340214.txt">Table of n, a(n) for n = 1..910</a>

%e For n=4, A340212(30)=8 and this is the first appearance of 8 in A340212, so a(4)=30.

%p V:= Vector(60): count:= 0:

%p q:= 3: r:= 5:

%p for i from 3 while count < 60 do

%p   p:= q; q:= r; r:= nextprime(r);

%p   for k from 2 by 2 do

%p     if isprime(k*q+p) and isprime(k*q+r) then break fi;

%p   od;

%p   v:= k/2;

%p   if v <= 60 and V[v] = 0 then count:= count+1; V[v]:= i; fi

%p od:

%o (PARI) f(n) =  my(p=prime(n), k=1); while (! (isprime(k*p+precprime(p-1)) && isprime(k*p+nextprime(p+1))), k++); k; \\ A340212

%o a(n) = my(m=3); while (f(m) != 2*n, m++); m; \\ _Michel Marcus_, Jan 01 2021

%Y Cf. A340211, A340212.

%K nonn

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, Dec 31 2020