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A165803
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Integers n such that the trajectory of n under repeated applications of the map k->(k-3)/2 is a chain of primes that reaches 2 or 3 (n itself need not be a prime).
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2
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OFFSET
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1,1
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COMMENTS
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For initial values n > 3, the map is applied at least once, so 9 is in the sequence although it is not a prime. The sequence consists of p = 2 and p = 3 and the two finite chains of primes that are formed by repeated application of p -> 2*p + 3, which are 2 -> 7 -> 17 -> 37 -> 77 and 3 -> 9.
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LINKS
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EXAMPLE
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(77-3)/2 = 37 (prime); (37-3)/2 = 17 (prime); (17-3)/2 = 7 (prime); (7-3)/2 = 2; stop (because 2 has been reached).
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MATHEMATICA
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f[n_] := Module[{k = n}, While[k > 3, k = (k - 3)/2; If[ !PrimeQ[k], Break[]]]; PrimeQ[k]]; A165803 = {}; Do[If[f[n], AppendTo[A165803, n]], {n, 5!}]; A165803
cpQ[n_]:=AllTrue[Rest[NestWhileList[(#-3)/2&, n, #!=2&&#!=3&, 1, 20]], PrimeQ]; Select[Range[100], cpQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jul 24 2019 *)
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CROSSREFS
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KEYWORD
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nonn,fini,full
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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