A115908 Numbers k such that phi(k) + prime(k) is a triangular number.

1, 5, 6, 15, 23, 141, 150, 258, 317, 340, 359, 378, 471, 503, 768, 1363, 2175, 2516, 3285, 3342, 3815, 3905, 4385, 4748, 5385, 5976, 6026, 6900, 7048, 7151, 7411, 7698, 8251, 8310, 8828, 10182, 10461, 12022, 12508, 13722, 13840, 15456, 16582

Offset: 1

Author

Giovanni Resta, Feb 06 2006

Keywords

• nonn

Examples

			phi(503) + prime(503) = 4095 = T(90).
		

Links

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

Crossrefs

Cf. A000010, A115882, A115907.

Programs

Mathematica

With[{trnos = Accumulate[Range[5000]]}, Select[Range[20000], MemberQ[trnos, (EulerPhi[#] + Prime[#])] &]] (* Harvey P. Dale, Jan 12 2011 *)
Select[Range[20000],OddQ[Sqrt[8(EulerPhi[#]+Prime[#])+1]]&] (* Harvey P. Dale, Dec 08 2024 *)

PARI

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