A283802 - OEIS

%I #21 Mar 03 2024 10:15:42

%S 21,73,132

%N Numbers k such that the concatenation of the first k odd composite numbers is a prime.

%C Indices k for which A283801(k) is prime.

%C A283801(21) = 91521252733353945495155576365697577818587 is a 41-digit prime; A283801(73) = 91521...247249253 is a 193-digit prime; A283801(132) = 91521...423425427 is a 370-digit prime.

%C Next term, if it exists, will be more than 5028.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive Number Sequences</a>

%t 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 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 Cf. A007908, A019518, A046035, A069151, A071904, A283801.

%K base,nonn,bref,more

%O 1,1

%A _XU Pingya_, Mar 17 2017