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 A087606 Smallest k such that n times concatenation of k with itself followed by a 9 is a prime, or 0 if no such number exists. 7
 1, 2, 0, 1, 1, 0, 1, 11, 0, 64, 5, 0, 2, 31, 0, 1, 5, 0, 10, 65, 0, 41, 212, 0, 5, 79, 0, 41, 160, 0, 5, 94, 0, 8, 82, 0, 23, 43, 0, 40, 26, 0, 391, 119, 0, 212, 4, 0, 1, 160, 0, 134, 28, 0, 208, 50, 0, 248, 35, 0, 113, 43, 0, 79, 7, 0, 70, 170, 0, 64, 94, 0, 19, 86, 0, 10, 118, 0, 34, 98 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Conjecture: a(3n) = 0. No other term is zero. a(3n)=0: consider the sum of the digits modulo 3. For the same reason, if a(m) is divisible by 3 then a(m)=0. - Sam Alexander, Nov 15 2003 LINKS XIAO Gang, Factoris - a program that factorizes huge integers, 1997-1999 EXAMPLE a(2) = 2 as 229 is a prime. but 119 is not. MATHEMATICA s[b_]:=(v={}; l=Length[b]; Do[v=Join[v, IntegerDigits[b[[k]]]], {k, l}]; v); a[n_]:=If[Mod[n, 3]!= 0, (For[m = 1, ! PrimeQ[10*FromDigits[s[Table[m, {n}]]] +9], m++ ]; m), 0]; Table[a[n], {n, 90}] (Firoozbakht) CROSSREFS Cf. A086920, A087604, A087605, A087607, A087608, A087609, A087610. Sequence in context: A083731 A216266 A177416 * A271368 A116799 A057556 Adjacent sequences:  A087603 A087604 A087605 * A087607 A087608 A087609 KEYWORD base,nonn AUTHOR Amarnath Murthy, Sep 18 2003 EXTENSIONS More terms from Sam Alexander, Nov 15 2003 More terms from Farideh Firoozbakht, Feb 04 2005 STATUS approved

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Last modified June 20 20:08 EDT 2021. Contains 345231 sequences. (Running on oeis4.)