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 A172024 a(n) = smallest number > a(n-1) such that a(1)*a(2)*...*a(n) + 1 or a(1)*a(2)*...*a(n) - 1 is prime. 1
 1, 2, 3, 4, 7, 9, 10, 11, 12, 13, 14, 15, 16, 20, 21, 23, 26, 27, 30, 35, 36, 37, 39, 40, 43, 53, 55, 67, 72, 85, 97, 98, 123, 130, 131, 132, 138, 141, 146, 180, 182, 185, 188, 192, 201, 225, 231, 236, 240, 248, 252, 254, 276, 300, 322, 326, 346, 372, 401, 413, 424 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Iain Fox, Table of n, a(n) for n = 1..500 MATHEMATICA For[n = 2; res = {1}; z = 1, n <= 1000, n++, If[PrimeQ[n z + 1] || PrimeQ[n z - 1], AppendTo[res, n]; z *= n]] res (* J. Mulder (jasper.mulder(AT)planet.nl), Jan 28 2010 *) nxt[{prd_, a_}]:=Module[{c=a+1}, While[ NoneTrue[prd*c+{1, -1}, PrimeQ], c++]; {prd*c, c}]; NestList[nxt, {1, 1}, 60][[All, 2]] (* Harvey P. Dale, Jan 12 2022 *) PROG (PARI) first(n) = { my(res = vector(n), p=1); res[1]=1; for(x=2, n, my(k=res[x-1]+1); while(!ispseudoprime(p*k+1) && !ispseudoprime(p*k-1), k++); res[x]=k; p*=k); res; } \\ Iain Fox, Nov 30 2017 CROSSREFS Sequence in context: A256606 A101128 A182802 * A285500 A119345 A086391 Adjacent sequences:  A172021 A172022 A172023 * A172025 A172026 A172027 KEYWORD nonn AUTHOR G. L. Honaker, Jr., Jan 22 2010 EXTENSIONS Terms beyond a(15) from R. J. Mathar and J. Mulder (jasper.mulder(AT)planet.nl), Jan 25 2010 STATUS approved

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Last modified August 9 00:15 EDT 2022. Contains 356016 sequences. (Running on oeis4.)