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 A247483 #11 Oct 30 2024 19:30:13
%S A247483 4,141,41414,1414141,314141413,33141414133,3331414141333,
%T A247483 14333141414133341,121433314141413334121,1012143331414141333412101,
%U A247483 17101214333141414133341210171,3171012143331414141333412101713,19317101214333141414133341210171391
%N A247483 a(1)=4; for n>=1, a(n+1) is the smallest palindromic semiprime with a(n) as a central substring.
%e A247483 a(1) = 4 = 2*2;
%e A247483 a(2) = 141 = 3*47;
%e A247483 a(3) = 41414 = 2*20707;
%e A247483 a(4) = 1414141 = 43*32887.
%t A247483 d[n_] := IntegerDigits[n]; t = {x = 4}; Do[i = 1; While[!PrimeOmega[y = FromDigits[Flatten[{z = d[i], d[x], Reverse[z]}]]]==2, i++]; AppendTo[t, x = y], {n, 12}]; t
%t A247483 nxt[n_]:=Module[{k=1},While[PrimeOmega[FromDigits[Join[IntegerDigits[k],IntegerDigits[n],Reverse[IntegerDigits[k]]]]]!=2,k++]; FromDigits[ Join[IntegerDigits[k],IntegerDigits[n],Reverse[IntegerDigits[k]]]]]; NestList[nxt,4,15] (* _Harvey P. Dale_, Oct 30 2024 *)
%Y A247483 Cf. A001358, A053600.
%K A247483 nonn,base
%O A247483 1,1
%A A247483 _Michel Lagneau_, Dec 01 2014