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 A075598 a(1) = 5 and then the smallest prime that is obtained by placing digits on both sides of the previous term. Or smallest prime that encompasses a(n-1). 8
 5, 151, 11519, 2115193, 121151939, 21211519397, 4212115193971, 342121151939719, 43421211519397199, 2434212115193971993, 224342121151939719937, 122434212115193971993787, 51224342121151939719937871, 2512243421211519397199378719, 325122434212115193971993787197, 93251224342121151939719937871973 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Robert Israel, Table of n, a(n) for n = 1..330 MAPLE f:= proc(n) local m, d, d1, v, x, y, y0, z, found;   m:= ilog10(n);   v:= infinity;   for d from 2 do     for d1 from 1 to d-1 do       found:= false;       for x from 10^(d1-1) to 10^d1-1 while not found do         if d-d1=1 then y0:= 1 else y0:= 10^(d-d1-1)+1 fi;         for y from y0 to 10^(d-d1)-1 by 2 do           z:= y+10^(d-d1)*n + 10^(d-d1+m+1)*x;           if isprime(z) then v:= min(v, z); found:= true; break fi       od od;     od;     if v < infinity then return v fi   od end proc: A[1]:= 5: for n from 2 to 20 do   A[n]:= f(A[n-1]) od: seq(A[n], n=1..20); # Robert Israel, Aug 28 2018 CROSSREFS Cf. A075595, A075596, A075597, A075599, A075600, A075601, A075602. Sequence in context: A082623 A332115 A262655 * A214691 A261822 A105230 Adjacent sequences:  A075595 A075596 A075597 * A075599 A075600 A075601 KEYWORD base,nonn AUTHOR Amarnath Murthy, Sep 28 2002 EXTENSIONS More terms from Sascha Kurz, Jan 20 2003 Terms a(13) and beyond from Robert Israel, Aug 28 2018 STATUS approved

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Last modified December 5 11:56 EST 2021. Contains 349557 sequences. (Running on oeis4.)