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 A283802 #32 Jun 16 2025 08:48:17
%S A283802 21,73,132
%N A283802 Numbers k such that the concatenation of the first k odd composite numbers is a prime.
%C A283802 Indices k for which A283801(k) is prime.
%C A283802 A283801(21) = 91521252733353945495155576365697577818587 is a 41-digit prime;
%C A283802 A283801(73) = 91521...247249253 is a 193-digit prime;
%C A283802 A283801(132) = 91521...423425427 is a 370-digit prime.
%C A283802 Next term, if it exists, will be more than 5028.
%C A283802 a(4) > 25000, if it exists. - _Michael S. Branicky_, Apr 30 2025
%H A283802 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive Number Sequences</a>
%t A283802 k = 2; cc = oc = 0; lst = {}; While[k < 428, If[OddQ@k && !PrimeQ@k, cc = cc*10^IntegerLength@k + k; oc++; If[PrimeQ[cc], AppendTo[lst, oc]]]; k++]; lst (* _Robert G. Wilson v_, Mar 18 2017 *)
%t A283802 Module[{nn=501,ocm},ocm=Select[Range[9,nn,2],CompositeQ];Select[ Range[ Length[ ocm]],PrimeQ[FromDigits[Flatten[IntegerDigits/@Take[ocm,#]]]]&]] (* _Harvey P. Dale_, Sep 02 2022 *)
%Y A283802 Cf. A007908, A019518, A046035, A069151, A071904, A283801.
%K A283802 base,nonn,bref,more
%O A283802 1,1
%A A283802 _XU Pingya_, Mar 17 2017