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

1,2

Links

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

Example

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

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 *)

Prog

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

Crossrefs

Cf. A000010, A000566, A051953.

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

Sequence in context: A319319 A003277 A322833 * A121615 A097605 A143578

Adjacent sequences: A117284 A117285 A117286 * A117288 A117289 A117290

Keyword

nonn

Author

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

Extensions

Offset corrected by Amiram Eldar, Mar 23 2021

Status

approved