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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
Sequence in context: A007126 A009576 A009580 * A117813 A012758 A208822
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 February 29 11:59 EST 2024. Contains 370425 sequences. (Running on oeis4.)