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Triangular numbers T such that T-2 is a prime.
1

%I #6 Mar 15 2023 11:02:12

%S 15,21,45,55,91,105,153,231,253,351,435,465,595,703,741,861,1035,1225,

%T 1431,1485,1711,1891,1953,2145,2701,3003,3081,3321,3741,4005,4095,

%U 4465,4753,5565,5671,6555,7021,7875,8515,9045,10011,10153,10731,11175,11781

%N Triangular numbers T such that T-2 is a prime.

%e 15-2 = 13, 21-2 = 19, 45-2 = 43, ...

%t Select[s=0;Table[n*(n+1)/2,{n,6!}],PrimeQ[ #-2]&]

%t Select[Accumulate[Range[200]],PrimeQ[#-2]&] (* _Harvey P. Dale_, Apr 09 2018 *)

%Y Cf. A000217, A129545, A129752.

%K nonn

%O 1,1

%A _Vladimir Joseph Stephan Orlovsky_, Dec 11 2009