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%I #13 Mar 23 2021 05:20:07
%S 1,2,3,5,7,9,10,11,13,17,19,23,29,31,37,39,41,43,45,47,52,53,54,55,57,
%T 59,61,67,68,71,73,79,83,85,89,90,97,101,103,107,109,113,114,117,127,
%U 129,131,137,139,149,151,157,159,163,167,168,173,175,179,181,191,193
%N Numbers k for which the cototient k-phi(k) is a triangular number.
%H Amiram Eldar, <a href="/A117284/b117284.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1001 from Harvey P. Dale)
%e 39 is in the sequence because 39-phi(39) = 15, which is a triangular number.
%t Select[Range[200],OddQ[Sqrt[8(#-EulerPhi[#])+1]]&] (* _Harvey P. Dale_, Jun 29 2015 *)
%o (PARI) isok(n) = ispolygonal(n - eulerphi(n), 3); \\ _Michel Marcus_, Feb 26 2014
%Y Cf. A000010, A000217, A051953.
%Y Cf. A117283, A117285, A117286, A117287, A117288, A117289.
%K nonn
%O 1,2
%A Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
%E Offset corrected by _Amiram Eldar_, Mar 23 2021
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