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 A125839 a(n) is the smallest number m such that k*m - 1 is prime for all k=3,4,...,n. 4
 1, 1, 6, 18, 120, 1260, 1485540, 28667100, 28667100, 842889105240, 2281585556250, 163881570370980, 45187548280664790 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,3 COMMENTS For n > 6, 10 divides a(n). LINKS Carlos Rivera, Problems & puzzles. EXAMPLE a(11)=28667100 because 3*28667100-1, 4*28667100-1, 5*28667100-1, 6*28667100-1, 7*28667100-1, 8*28667100-1, 9*28667100-1, 10*28667100-1 & 11*28667100-1 are prime and 28667100 is the smallest number with this property. MATHEMATICA a[n_] := Block[{k = If[n<5, 1, 6], s}, s = k;  While[! AllTrue[k Range[3, n] - 1, PrimeQ], k += s]; k]; a /@ Range[3, 9] (* Giovanni Resta, Mar 29 2017 *) CROSSREFS Cf. A125838, A088651, A088250. Sequence in context: A007126 A009576 A009580 * A117813 A012758 A208822 Adjacent sequences:  A125836 A125837 A125838 * A125840 A125841 A125842 KEYWORD hard,more,nonn AUTHOR Luke Pebody (luke.pebody(AT)gmail.com), Jan 02 2007 EXTENSIONS a(12)-a(13) from Farideh Firoozbakht a(14)-a(15) from Giovanni Resta, Mar 30 2017 STATUS approved

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Last modified May 22 14:32 EDT 2019. Contains 323480 sequences. (Running on oeis4.)