A115905 Numbers k such that k + phi(k) is a triangular number.

2, 4, 9, 11, 20, 23, 25, 27, 49, 53, 92, 93, 105, 121, 127, 147, 150, 163, 169, 220, 228, 231, 233, 281, 286, 289, 294, 332, 361, 431, 474, 529, 541, 543, 548, 613, 660, 675, 710, 726, 743, 825, 827, 841, 888, 891, 961, 977, 993, 1014, 1028, 1265, 1310, 1369

Offset

1,1

Links

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

Example

121 + phi(121) = 231 = T(21).

Mathematica

Select[Range[1500], OddQ[Sqrt[8(#+EulerPhi[#])+1]]&] (* Harvey P. Dale, May 11 2015 *)

Prog

(PARI) isok(n) = ispolygonal(n + eulerphi(n), 3); \\ Michel Marcus, Jan 25 2014

Crossrefs

Cf. A000010, A000217, A069066, A115908, A115910.

Sequence in context: A186699 A093859 A266257 * A292769 A307997 A096134

Adjacent sequences: A115902 A115903 A115904 * A115906 A115907 A115908

Keyword

nonn

Author

Giovanni Resta, Feb 06 2006

Status

approved