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A003275
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Values of phi(n) = phi(n+1).
(Formerly M1874)
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5
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1, 2, 8, 48, 80, 96, 128, 240, 288, 480, 1008, 1200, 1296, 1440, 1728, 2592, 2592, 4800, 5600, 6480, 8640, 11040, 12480, 14976, 19008, 19200, 22464, 24320, 24576, 21120, 28416, 27840, 25920, 32000, 32768, 36000, 47520, 52992, 60480, 59904, 79200, 89280, 96768
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OFFSET
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1,2
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COMMENTS
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In other words, consider n = 1,2,3,4,..., and if phi(n)=phi(n+1), add phi(n) to the sequence.
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REFERENCES
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R. K. Guy, Unsolved Problems Number Theory, Sect. B36.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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K. Miller, Solutions of phi(n) = phi(n+1) for 1 <= n <= 500000. Unpublished, 1972. [ See Review, Math. Comp., Vol. 27, p. 447-448, 1973 ].
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FORMULA
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MATHEMATICA
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Cases[Split[Table[EulerPhi[k], {k, 1, 50000}]], {_, _}][[1;; 27, 1]] (* Jean-François Alcover, Mar 20 2011 *)
#[[1]]&/@Select[Partition[EulerPhi[Range[80000]], 2, 1], #[[1]]==#[[2]]&] (* Harvey P. Dale, Oct 03 2012 *)
SequenceCases[EulerPhi[Range[200000]], {x_, x_}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 05 2019 *)
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PROG
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(Haskell)
a003275 = a000010 . fromIntegral . a001274
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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STATUS
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approved
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