This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A228150 #11 Jun 13 2021 03:29:43
%S A228150 7,19,199,1009,19417,160009,1086791,10301299,139993999,1391909989,
%T A228150 10597399381,108979916999,1993999651909,13924969999799
%N A228150 a(n) is the smallest n-digit prime which remains prime whenever a 9 is inserted, appended or prepended, or 0 if no such prime exists.
%C A228150 Smallest n-digit term of A215421.
%e A228150 a(4) = 1009 is the smallest 4-digit prime which yields 5 primes by inserting a 9 in every possible position: 91009, 19009, 10909, 10099, 10099.
%t A228150 pp[n_, k_] := Catch[Block[{d = IntegerDigits@n}, Do[If[! PrimeQ[ FromDigits[ Insert[d, k, i]]], Throw[False]], {i, 1+Length@d}]; True]]; a[n_] := Catch[ Block[{p = NextPrime[10^(n-1)]}, While[p < 10^n, If[pp[p, 9], Throw@p, p = NextPrime@p]]; 0]]; a /@ Range[8]
%Y A228150 Cf. A215421, A164328, A228148, A228149.
%K A228150 nonn,base,more
%O A228150 1,1
%A A228150 _Giovanni Resta_, Aug 13 2013