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A003275 Values of phi(n) = phi(n+1).
(Formerly M1874)
5
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

In other words, consider n = 1,2,3,4,..., and if phi(n)=phi(n+1), add phi(n) to the sequence.

REFERENCES

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).

LINKS

T. D. Noe, Table of n, a(n) for n=1..2567

K. Miller, The equation phi(n) = phi(n+1), Unpublished M.S., ND..

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 Unpublished, 1972.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 ].

K. Miller, Solutions of phi(n) = phi(n+1) for 1 <= n <= 500000, Mathematics of Computation 27 (1973), 47-48. (Annotated scanned copy)

Leo Moser, Some equations involving Euler's totient function, Amer. Math. Monthly, 56 (1949), 22-23.

Eric Weisstein's World of Mathematics, Totient Function

FORMULA

a(n) = A000010(A001274(n)). - Reinhard Zumkeller, May 20 2014

MATHEMATICA

Cases[Split[Table[EulerPhi[k], {k, 1, 50000}]], {_, _}][[1;; 27, 1]] (* Jean-Fran├žois 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 *)

PROG

(Haskell)

a003275 = a000010 . fromIntegral . a001274

-- Reinhard Zumkeller, May 20 2014

CROSSREFS

Cf. A000010, A001274.

Sequence in context: A233337 A199136 A181413 * A253665 A078558 A003032

Adjacent sequences:  A003272 A003273 A003274 * A003276 A003277 A003278

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified October 16 03:37 EDT 2019. Contains 328040 sequences. (Running on oeis4.)