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 A117286 Numbers k for which the cototient k-phi(k) is a hexagonal number. 7
 1, 2, 3, 5, 7, 10, 11, 13, 17, 19, 23, 29, 31, 37, 39, 41, 43, 47, 52, 53, 55, 59, 61, 67, 71, 73, 79, 83, 89, 90, 97, 101, 103, 107, 109, 113, 117, 127, 129, 131, 137, 139, 149, 151, 157, 163, 167, 168, 173, 179, 181, 191, 193, 197, 199, 200, 205, 211, 223, 227, 229 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 55 is in the sequence because 55-phi(55) = 15, which is a hexagonal number. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 MATHEMATICA 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 *) PROG (PARI) isok(n) = ispolygonal(n - eulerphi(n), 6); \\ Michel Marcus, Feb 26 2014 CROSSREFS Cf. A000010, A000384, A051953. Cf. A117283, A117284, A117285, A117287, A117288, A117289. Sequence in context: A064627 A324813 A235050 * A169802 A194948 A266679 Adjacent sequences: A117283 A117284 A117285 * A117287 A117288 A117289 KEYWORD nonn AUTHOR Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006 EXTENSIONS Corrected by T. D. Noe, Oct 25 2006 STATUS approved

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Last modified September 12 19:13 EDT 2024. Contains 375853 sequences. (Running on oeis4.)