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A063920 Numbers n such that n = 2*phi(n) + phi(phi(n)). 2
10, 14, 20, 28, 40, 56, 80, 112, 160, 224, 320, 448, 640, 896, 1280, 1792, 2560, 3584, 5120, 7168, 10240, 14336, 20480, 28672, 40960, 57344, 81920, 114688, 163840, 229376, 327680, 458752, 655360, 917504, 1310720, 1835008, 2621440, 3670016, 5242880, 7340032, 10485760 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Previous name was: t(n) = z(n) where t(n)= |eulerphi(n)-n| and z(n)= t(t(n)-n).

LINKS

Table of n, a(n) for n=0..40.

Ralf Stephan, Prove or disprove: 100 conjectures from the OEIS, arXiv:math/0409509 [math.CO], 2004.

Lawrence Sze, Conjecture 36 (at archive.org).

Lawrence Sze, Conjecture 36 - from OEIS - a.k.a. A063920, preprint, 2004. [cached copy]

Index entries for linear recurrences with constant coefficients, signature (0,2)

FORMULA

G.f.: (10 + 14x)/(1 - 2x^2).

a(n) = (12-2*(-1)^n) * 2^floor(n/2). - Ralf Stephan, Jul 19 2013

MATHEMATICA

CoefficientList[Series[(10 + 14 x) / (1 - 2 x^2), {x, 0, 40}], x] (* Vincenzo Librandi, Feb 29 2016 *)

PROG

(PARI) t(n) = abs(eulerphi(n)-n); z(n) = t(t(n)-n);

for(n=1, 113, if(t(n)==z(n), print1(n, ", ")))

(MAGMA) [(12-2*(-1)^n)*2^Floor(n/2): n in [0..50]]; // Vincenzo Librandi, Feb 29 2016

CROSSREFS

Cf. A070875 (the same sequence, if we omit the two initial terms).

Sequence in context: A031274 A272375 A246473 * A269703 A057487 A073486

Adjacent sequences:  A063917 A063918 A063919 * A063921 A063922 A063923

KEYWORD

nonn

AUTHOR

Jason Earls, Aug 31 2001

EXTENSIONS

Better name from Ivan Neretin, Feb 28 2016

STATUS

approved

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Last modified September 23 01:32 EDT 2020. Contains 337291 sequences. (Running on oeis4.)