A115908 - OEIS

%I #22 Dec 08 2024 11:09:39

%S 1,5,6,15,23,141,150,258,317,340,359,378,471,503,768,1363,2175,2516,

%T 3285,3342,3815,3905,4385,4748,5385,5976,6026,6900,7048,7151,7411,

%U 7698,8251,8310,8828,10182,10461,12022,12508,13722,13840,15456,16582

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

%H Amiram Eldar, <a href="/A115908/b115908.txt">Table of n, a(n) for n = 1..10000</a>

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

%t With[{trnos = Accumulate[Range[5000]]}, Select[Range[20000], MemberQ[trnos, (EulerPhi[#] + Prime[#])] &]] (* _Harvey P. Dale_, Jan 12 2011 *)

%t Select[Range[20000],OddQ[Sqrt[8(EulerPhi[#]+Prime[#])+1]]&] (* _Harvey P. Dale_, Dec 08 2024 *)

%o (PARI) isok(n) = ispolygonal(prime(n) + eulerphi(n), 3); \\ _Michel Marcus_, Jan 25 2014

%Y Cf. A000010, A115882, A115907.

%K nonn

%O 1,2

%A _Giovanni Resta_, Feb 06 2006