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 A117288 #17 Jul 08 2024 08:29:09 %S A117288 1,2,3,5,7,11,12,13,14,16,17,19,23,29,31,37,41,43,45,47,53,57,59,61, %T A117288 67,71,73,76,79,83,85,89,97,101,103,107,109,113,127,131,137,139,144, %U A117288 149,151,157,160,163,167,173,176,179,181,184,188,191,193,197,199,211,223 %N A117288 Numbers k for which the cototient k-phi(k) is an octagonal number. %H A117288 Amiram Eldar, <a href="/A117288/b117288.txt">Table of n, a(n) for n = 1..10000</a> %e A117288 45 is in the sequence because 45-phi(45) = 21, which is an octagonal number. %t A117288 octQ[n_] := n == 0 || IntegerQ[(Sqrt[48*n + 16] + 4)/12]; Select[Range[250], octQ[# - EulerPhi[#]] &] (* _Amiram Eldar_, Mar 23 2021 *) %o A117288 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 8); \\ _Michel Marcus_, Feb 26 2014 %Y A117288 Cf. A000010, A000567, A051953. %Y A117288 Cf. A117283, A117284, A117285, A117286, A117287, A117289. %K A117288 nonn %O A117288 1,2 %A A117288 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006 %E A117288 Offset corrected by _Amiram Eldar_, Mar 23 2021