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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)



Robert Israel, Table of n, a(n) for n = 1..330


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;


    if v < infinity then return v fi


end proc:

A[1]:= 5:

for n from 2 to 20 do

  A[n]:= f(A[n-1])


seq(A[n], n=1..20); # Robert Israel, Aug 28 2018


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




Amarnath Murthy, Sep 28 2002


More terms from Sascha Kurz, Jan 20 2003

Terms a(13) and beyond from Robert Israel, Aug 28 2018



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