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

1,2

Links

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

Example

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

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 *)

Prog

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

Crossrefs

Cf. A000010, A115882, A115907.

Sequence in context: A030533 A329634 A227066 * A247962 A241307 A185876

Adjacent sequences: A115905 A115906 A115907 * A115909 A115910 A115911

Keyword

nonn

Author

Giovanni Resta, Feb 06 2006

Status

approved