A117287 Numbers k for which the cototient k-phi(k) is a heptagonal number.

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

Offset: 1

Author

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

Keywords

• nonn

Examples

			49 is in the sequence because 49-phi(49) = 7, which is a heptagonal number.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A000566, A051953.

Cf. A117283, A117284, A117285, A117286, A117288, A117289.

Programs

Mathematica

hnQ[n_]:=Module[{c=n-EulerPhi[n]},IntegerQ[(3+Sqrt[9+40c])/10]]; Join[{1},Select[Range[0,250],hnQ]] (* Harvey P. Dale, Mar 26 2011 *)

PARI

isok(n) = ispolygonal(n - eulerphi(n), 7); \\ Michel Marcus, Feb 26 2014

Extensions

Offset corrected by Amiram Eldar, Mar 23 2021