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%I #32 Sep 08 2022 08:46:15
%S 7,13,19,37,73,91,97,109,163,193,433,487,577,703,769,793,925,1153,
%T 1297,1459,2593,2917,3457,3889,4699,5551,6697,7999,8701,10369,10591,
%U 11803,12289,16471,17497,18433,33251,39367,52489,56791,79249,124357,127927,137899
%N Numbers n such that phi(n) = 3*phi(n-1).
%C Prime terms are in A058383.
%C See A266276(n) = the smallest numbers k such that phi(k) = n * phi(k-1) for n >=1: 2, 3, 7, 1261, 11242771, ...
%C Number of terms < 10^k: 1, 7, 17, 29, 41, 86, 205, 446, 1001, 2295, ..., . - _Robert G. Wilson v_, Jan 24 2016
%C All terms are == +-1 (mod 6) but mostly 1 (> 95%). - _Robert G. Wilson v_, Jan 24 2016
%H Robert G. Wilson v, <a href="/A266268/b266268.txt">Table of n, a(n) for n = 1..2467</a> (first 405 terms from G. C. Greubel)
%F a(n) = A067143(n) + 1.
%e 19 is in the sequence because phi(19) = 18 = 3*phi(18) = 3*6.
%t Select[Range[5000], EulerPhi[ # ]==3*EulerPhi[ #-1]&] (* _G. C. Greubel_, Dec 26 2015 *)
%o (Magma) [n: n in [2..2*10^5] | EulerPhi(n) eq 3*EulerPhi(n-1)]; // _Vincenzo Librandi_, Dec 26 2015
%o (PARI) isok(n) = eulerphi(n) == 3*eulerphi(n-1); \\ _Michel Marcus_, Dec 27 2015
%o (PARI) lista(nn) = for(n=1, nn, if(eulerphi(n) == 3*eulerphi(n-1), print1(n, ", "))); \\ _Altug Alkan_, Jan 24 2016
%Y Cf. A000010, A058383, A171271 (numbers n such that phi(n) = 2*phi(n-1)), A266276.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Dec 26 2015
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