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 A071407 Least k such that k*prime(n) + 1 and k*prime(n) - 1 are twin primes. 5
 2, 2, 6, 6, 18, 24, 6, 12, 6, 12, 42, 54, 30, 24, 6, 120, 18, 258, 24, 18, 84, 132, 54, 48, 114, 42, 6, 6, 48, 24, 144, 30, 6, 12, 12, 78, 24, 36, 30, 54, 132, 18, 90, 36, 66, 18, 42, 30, 120, 30, 36, 42, 18, 18, 54, 84, 60, 12, 210, 12, 6, 60, 150, 102, 6, 210, 30, 24, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Note that 6 divides a(n) for n > 2. - T. D. Noe, Jan 07 2013 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 EXAMPLE n=4: prime(4)=7, a(4)=6 because 6*prime(4)=42 and {41,43} are primes. MATHEMATICA Table[fl=1; Do[s=(Prime[j])*k; If[PrimeQ[s-1]&&PrimeQ[s+1]&&Equal[fl, 1], Print[{j, k}]; fl=0], {k, 1, 2*j^2}], {j, 0, 100}] PROG (Haskell) a071407 n = head [k | k <- [2, 4..], let x = k * a000040 n, a010051' (x - 1) == 1, a010051' (x + 1) == 1] -- Reinhard Zumkeller, Feb 14 2013 CROSSREFS Cf. A071256, A071404-A071406, A060256, A060210. Cf. A071558 (k at every integer). Cf. A220141, A220142 (record values). Sequence in context: A011260 A117855 A086442 * A309094 A109859 A128057 Adjacent sequences: A071404 A071405 A071406 * A071408 A071409 A071410 KEYWORD nonn AUTHOR Labos Elemer, May 24 2002 STATUS approved

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Last modified March 29 19:01 EDT 2023. Contains 361599 sequences. (Running on oeis4.)