|
%I #9 Apr 15 2018 10:42:55
%S 5,7,15,96
%N sigma(n) times the n-th prime gives a triangular number.
%C Probably finite.
%C No further terms up to n = 1 million. - _Harvey P. Dale_, Apr 15 2018
%e prime(5) * sigma(5) = 11 * 6 = T(12).
%e prime(7) * sigma(7) = 17 * 8 = T(18).
%e prime(15) * sigma(15) = 47 * 24 = T(48).
%e prime(96) * sigma(96) = 503 * 252 = T(503).
%t Select[Range[200],OddQ[Sqrt[8(Prime[#]DivisorSigma[1,#])+1]]&] (* _Harvey P. Dale_, Apr 15 2018 *)
%o (PARI) isok(n) = ispolygonal(sigma(n)*prime(n), 3); \\ _Michel Marcus_, Jul 05 2014
%K nonn
%O 1,1
%A _Giovanni Resta_, Feb 13 2006
| 