A117284 Numbers k for which the cototient k-phi(k) is a triangular number.

1, 2, 3, 5, 7, 9, 10, 11, 13, 17, 19, 23, 29, 31, 37, 39, 41, 43, 45, 47, 52, 53, 54, 55, 57, 59, 61, 67, 68, 71, 73, 79, 83, 85, 89, 90, 97, 101, 103, 107, 109, 113, 114, 117, 127, 129, 131, 137, 139, 149, 151, 157, 159, 163, 167, 168, 173, 175, 179, 181, 191, 193

Offset

1,2

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1001 from Harvey P. Dale)

Example

39 is in the sequence because 39-phi(39) = 15, which is a triangular number.

Mathematica

Select[Range[200], OddQ[Sqrt[8(#-EulerPhi[#])+1]]&] (* Harvey P. Dale, Jun 29 2015 *)

Prog

(PARI) isok(n) = ispolygonal(n - eulerphi(n), 3); \\ Michel Marcus, Feb 26 2014

Crossrefs

Cf. A000010, A000217, A051953.

Cf. A117283, A117285, A117286, A117287, A117288, A117289.

Sequence in context: A302403 A096738 A167857 * A137377 A274793 A339238

Adjacent sequences: A117281 A117282 A117283 * A117285 A117286 A117287

Keyword

nonn

Author

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

Extensions

Offset corrected by Amiram Eldar, Mar 23 2021

Status

approved