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A117286
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Numbers n for which the cototient n-phi(n) is a hexagonal number.
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0
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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; internal format)
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OFFSET
| 1,2
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COMMENTS
| 55 is in the sequence because 55-phi(55)=15, which is a hexagonal number.
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MATHEMATICA
| cthQ[n_]:=Module[{c=n-EulerPhi[n]}, IntegerQ[(Sqrt[1+8c]+1)/4]]; Join[{1}, Select[Range[250], cthQ]] (* From Harvey P. Dale, Jan 16 2012 *)
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CROSSREFS
| Cf. A000384.
Sequence in context: A058590 A187711 A064627 * A169802 A194948 A191211
Adjacent sequences: A117283 A117284 A117285 * A117287 A117288 A117289
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KEYWORD
| nonn
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AUTHOR
| Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
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EXTENSIONS
| Corrected by T. D. Noe (noe(AT)sspectra.com), Oct 25 2006
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