This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A117289 #18 Mar 23 2021 05:18:43 %S A117289 1,2,3,5,7,11,13,17,19,21,23,27,29,31,36,37,40,41,43,44,46,47,53,59, %T A117289 61,66,67,70,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139, %U A117289 149,151,157,163,167,173,179,181,191,193,197,199,207,211,219,223,227,229,231 %N A117289 Numbers k for which the cototient k-phi(k) is an enneagonal number (A001106). %H A117289 Robert Israel, <a href="/A117289/b117289.txt">Table of n, a(n) for n = 1..10000</a> %e A117289 44 is in the sequence because 44-phi(44) = 24, which is an enneagonal number. %p A117289 N:= 1000: # to get all terms <= N %p A117289 enneagonal:= [seq(n*(7*n-5)/2, n=0..floor((sqrt(25+56*N)+5)/14))]: %p A117289 select(t -> member(t - numtheory:-phi(t), enneagonal), [$1..N]); # _Robert Israel_, Mar 30 2018 %t A117289 ennQ[n_] := n == 0 || IntegerQ[(Sqrt[56*n + 25] + 5)/14]; Select[Range[250], ennQ[# - EulerPhi[#]] &] (* _Amiram Eldar_, Mar 23 2021 *) %o A117289 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 9); \\ _Michel Marcus_, Feb 26 2014 %Y A117289 Cf. A000010, A001106, A051953. %Y A117289 Cf. A117283, A117284, A117285, A117286, A117287, A117288. %K A117289 nonn %O A117289 1,2 %A A117289 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006 %E A117289 Offset changed to 1 by _Robert Israel_, Mar 30 2018