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A117287
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Numbers n for which the cototient n-phi(n) is a heptagonal number.
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0
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1, 2, 3, 5, 7, 11, 13, 15, 17, 19, 23, 29, 31, 34, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 159, 163, 167, 173, 175, 179, 181, 189, 191, 193, 196, 197, 199, 208, 211, 223, 227, 229, 233
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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EXAMPLE
| 49 is in the sequence because 49-phi(49)=7, which is a heptagonal number.
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MATHEMATICA
| hnQ[n_]:=Module[{c=n-EulerPhi[n]}, IntegerQ[(3+Sqrt[9+40c])/10]]; Join[{1}, Select[Range[0, 250], hnQ]] (* From Harvey P. Dale, Mar 26 2011 *)
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CROSSREFS
| Cf. A000566.
Sequence in context: A102553 A069161 A003277 * A121615 A097605 A143578
Adjacent sequences: A117284 A117285 A117286 * A117288 A117289 A117290
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KEYWORD
| nonn
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AUTHOR
| Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
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