Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #20 Nov 25 2019 03:29:01
%S 106,166,316,526,586,736,1156,1366,1485,1486,1576,1756,1786,1816,1996,
%T 2146,2206,2416,2476,2535,2536,2806,2836,3003,3004,3046,3316,3466,
%U 3675,3676,3886,3927,3928,4096,4126,4306,4455,4456,4485,4486,4516,4726,4785,4786
%N Numbers k such that phi(k-1) < phi(k) < phi(k+1), where phi is the Euler totient function (A000010).
%D Jean-Marie De Koninck, Those Fascinating Numbers, American Mathematical Society, 2009, page 34, entry 105.
%H Amiram Eldar, <a href="/A078776/b078776.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harvey P. Dale)
%e phi(105)=48 < phi(106)=52 < phi(107)=106, so 106 is in the sequence.
%t Flatten[Position[Partition[EulerPhi[Range[5000]],3,1],_?(Min[Differences[ #]]> 0&)]//Quiet]+1 (* _Harvey P. Dale_, Jun 14 2019 *)
%o (PARI) for (n=2,3000, if (eulerphi(n-1)<eulerphi(n) && eulerphi(n)<eulerphi(n+1),print1(n",")))
%Y Cf. A000010.
%K nonn
%O 1,1
%A _Jon Perry_, Jan 09 2003
%E More terms from _Michel Marcus_, Mar 17 2014