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A280450
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Smallest k > 2n+1 such that phi(k) = phi(2n+1).
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1
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4, 8, 9, 14, 22, 21, 16, 32, 27, 26, 46, 33, 38, 58, 62, 44, 39, 57, 45, 55, 49, 52, 94, 86, 64, 106, 75, 63, 118, 77, 74, 104, 134, 92, 142, 91, 82, 93, 158, 162, 166, 128, 116, 115, 95, 99, 111, 119, 122, 125, 206, 112, 214, 133, 117, 145, 178, 135, 153, 242
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OFFSET
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1,1
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COMMENTS
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All terms are composite.
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LINKS
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FORMULA
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2n+1 < a(n) < 4n+3.
a(n)=2n+2 if and only if 2n+1 is in A001274.
If n > 3 is in A005384, then a(n)=4n+2. (End)
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MAPLE
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f:= n -> min(select(`>`, numtheory:-invphi(numtheory:-phi(2*n+1)), 2*n+1)):
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MATHEMATICA
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Table[k = 2 n + 2; While[EulerPhi@ k != #, k++] &@ EulerPhi[2 n + 1]; k, {n, 120}] (* Michael De Vlieger, Jan 03 2017 *)
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PROG
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(PARI) a(n) = my(k=2*n+2); while(eulerphi(k)!=eulerphi(2*n+1), k++); k \\ Felix Fröhlich, Jan 05 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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