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 A323532 Numbers k such that the decimal concatenation of the numbers from 1 up to k followed by digit reversals of the numbers from (k-1) down to 1 is a prime. 0
 10, 586, 2219 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The definition is related to the sequence discussed by N. J. A. Sloane (in Notices of the AMS (2018), Vol. 65, No. 9, pp. 1070-71) for which only a(1)-a(2) are known. a(1) corresponds to a memorable prime (12345678910987654321); a(4) > 10000 (if it exists). LINKS N. J. A. Sloane, The On-Line Encyclopedia of Integer Sequences, Notices, Amer. Math. Soc., 65 (No. 9, Oct. 2018), 1062-1074. EXAMPLE 10 is a term because 12345678910987654321 is a prime. 2219 is a term because 1...22172218221981227122...1 is a 15534-digit probable prime (where 8122 following 2219 corresponds to the digit reversal of 2218, 7122 to that of 2217, etc. down to 1). MATHEMATICA a[n_]:=Block[{cn=Drop[FoldList[Append, {}, ToString/@Range@n], 2]}, ParallelMap[If[PrimeQ[FromDigits@@{#<>Reverse@StringReverse@Most@#}], Length@#, Nothing]&, cn]]; a PROG (PARI) f(n) = eval(concat(vector(2*n-1, k, if(k<=n, Str(k), concat(apply(x->Str(x), Vecrev(digits(2*n-k)))))))); isok(n) = ispseudoprime(f(n)); \\ Michel Marcus, Jan 20 2019 CROSSREFS Cf. A173426 (similar but different concatenation scheme). Sequence in context: A212925 A322654 A337342 * A273032 A185277 A006441 Adjacent sequences:  A323529 A323530 A323531 * A323533 A323534 A323535 KEYWORD nonn,base,more AUTHOR Mikk Heidemaa, Jan 17 2019 STATUS approved

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Last modified June 22 19:28 EDT 2021. Contains 345388 sequences. (Running on oeis4.)