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A056853
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Numbers n satisfying phi(n+1) - phi(n-1) = 2.
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2
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4, 6, 7, 12, 13, 15, 18, 19, 21, 30, 42, 45, 60, 63, 72, 93, 102, 108, 117, 138, 150, 165, 180, 192, 198, 213, 228, 240, 255, 270, 282, 312, 333, 348, 357, 420, 432, 453, 462, 522, 525, 570, 600, 618, 642, 660, 693, 717, 765, 810, 822, 828, 858, 882, 933, 957
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OFFSET
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1,1
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LINKS
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EXAMPLE
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phi(13+1)-phi(13-1) = 2, so 13 is a term of the sequence.
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MATHEMATICA
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Select[Range[10^3], EulerPhi[ # + 1] - EulerPhi[ # - 1] == 2 &]
Flatten[Position[Partition[EulerPhi[Range[1000]], 3, 1], _?(Last[#]-First[#] == 2&), {1}, Heads->False]]+1 (* Harvey P. Dale, Jul 14 2014 *)
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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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