

A117286


Numbers k for which the cototient kphi(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
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OFFSET

1,2


COMMENTS

55 is in the sequence because 55phi(55) = 15, which is a hexagonal number.


LINKS



MATHEMATICA

cthQ[n_]:=Module[{c=nEulerPhi[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



KEYWORD

nonn


AUTHOR

Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006


EXTENSIONS



STATUS

approved



