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%I #13 Mar 23 2021 05:19:26
%S 1,2,3,5,7,11,13,17,18,19,20,22,23,25,29,30,31,37,41,43,47,53,59,61,
%T 67,71,73,75,79,83,89,97,101,102,103,107,109,110,113,127,131,132,137,
%U 139,140,149,151,155,157,163,167,173,179,181,191,193,197,199,203,211,223
%N Numbers k for which the cototient k-phi(k) is a pentagonal number.
%H Amiram Eldar, <a href="/A117285/b117285.txt">Table of n, a(n) for n = 1..10000</a>
%e 30 is in the sequence because 30-phi(30) = 22, which is a pentagonal number.
%t pentQ[n_] := n == 0 || IntegerQ[(Sqrt[24*n + 1] + 1)/6]; Select[Range[250], pentQ[# - EulerPhi[#]] &] (* _Amiram Eldar_, Mar 23 2021 *)
%o (PARI) isok(n) = ispolygonal(n - eulerphi(n), 5); \\ _Michel Marcus_, Feb 26 2014
%Y Cf. A000010, A000326, A051953.
%Y Cf. A117283, A117284, 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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