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 A053989 Smallest k such that nk-1 is prime. 15
 3, 2, 1, 1, 4, 1, 2, 1, 2, 2, 4, 1, 8, 1, 2, 2, 4, 1, 2, 1, 2, 2, 6, 1, 6, 4, 2, 3, 6, 1, 2, 1, 4, 2, 4, 2, 2, 1, 6, 2, 4, 1, 6, 1, 2, 3, 6, 1, 2, 3, 2, 2, 4, 1, 2, 3, 2, 3, 6, 1, 8, 1, 4, 2, 6, 2, 6, 1, 2, 2, 4, 1, 14, 1, 2, 2, 4, 3, 2, 1, 8, 2, 4, 1, 6, 3, 2, 3, 16, 1, 2, 4, 6, 3, 4, 2, 2, 1, 2, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA a(n) = (A038700(n)+1)/n. EXAMPLE a(5)=4 because the smallest prime in the sequence 5k-1 (4,9,14,19,24...) is 19 when k=4 PROG (Haskell) a053989 n = head [k | k <- [1..], a010051' (k * n - 1) == 1] -- Reinhard Zumkeller, Feb 14 2013 (PARI) a(n)=my(j); while(!isprime(j++*n-1), ); j \\ Charles R Greathouse IV, Apr 18 2013 CROSSREFS Cf. A034693, A038700, A071558, A010051, A103689, A200996. Sequence in context: A188316 A197027 A143772 * A239938 A097794 A275494 Adjacent sequences:  A053986 A053987 A053988 * A053990 A053991 A053992 KEYWORD easy,nonn AUTHOR Henry Bottomley, Apr 04 2000 STATUS approved

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Last modified October 29 19:29 EDT 2020. Contains 338069 sequences. (Running on oeis4.)