|
%I #15 Mar 23 2021 05:19:18
%S 1,2,3,5,7,10,11,13,17,19,23,29,31,37,39,41,43,47,52,53,55,59,61,67,
%T 71,73,79,83,89,90,97,101,103,107,109,113,117,127,129,131,137,139,149,
%U 151,157,163,167,168,173,179,181,191,193,197,199,200,205,211,223,227,229
%N Numbers k for which the cototient k-phi(k) is a hexagonal number.
%C 55 is in the sequence because 55-phi(55) = 15, which is a hexagonal number.
%H Amiram Eldar, <a href="/A117286/b117286.txt">Table of n, a(n) for n = 1..10000</a>
%t cthQ[n_]:=Module[{c=n-EulerPhi[n]},IntegerQ[(Sqrt[1+8c]+1)/4]]; Join[{1}, Select[Range[250],cthQ]] (* _Harvey P. Dale_, Jan 16 2012 *)
%o (PARI) isok(n) = ispolygonal(n - eulerphi(n), 6); \\ _Michel Marcus_, Feb 26 2014
%Y Cf. A000010, A000384, A051953.
%Y Cf. A117283, A117284, A117285, A117287, A117288, A117289.
%K nonn
%O 1,2
%A Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
%E Corrected by _T. D. Noe_, Oct 25 2006
|
