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n+prime(n)+prime(prime(n)) is a triangular number, where prime(n) is the n-th prime.
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%I #9 Aug 11 2019 11:38:55

%S 1,2,4,210,381,573,1009,1094,1117,2139,3123,4471,6459,7197,12534,

%T 13177,13335,13603,14178,19479,21512,23427,31833,35965,37924,38446,

%U 39726,55375,58851,59232,61015,63849,64344,70471,75616,76770,79590

%N n+prime(n)+prime(prime(n)) is a triangular number, where prime(n) is the n-th prime.

%H Harvey P. Dale, <a href="/A116010/b116010.txt">Table of n, a(n) for n = 1..145</a>

%e 1009+prime(1009)+prime(prime(1009))=T(426).

%t Select[Range[80000],OddQ[Sqrt[1+8(#+Prime[#]+Prime[Prime[#]])]]&] (* _Harvey P. Dale_, Aug 11 2019 *)

%o (PARI) isok(n) = ispolygonal(n + prime(n) + prime(prime(n)), 3); \\ _Michel Marcus_, Jan 27 2014

%Y Cf. A116011.

%K nonn

%O 1,2

%A _Giovanni Resta_, Feb 13 2006