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A283802
Numbers k such that the concatenation of the first k odd composite numbers is a prime.
1
OFFSET
1,1
COMMENTS
Indices k for which A283801(k) is prime.
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.
Next term, if it exists, will be more than 5028.
LINKS
Eric Weisstein's World of Mathematics, Consecutive Number Sequences
MATHEMATICA
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 *)
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 *)
CROSSREFS
KEYWORD
base,nonn,bref,more
AUTHOR
XU Pingya, Mar 17 2017
STATUS
approved