This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A175117 #8 Feb 14 2021 16:26:10
%S A175117 3,9,55,181,215,459,1361,1417,1623,2047,2445,4685,6687,7353,7785,7925,
%T A175117 8489,10333,10515,11179,14087,15227,15829
%N A175117 Numbers k such that Sum_(i=1..k) prime(i)*(-1)^(i+1) is a square.
%C A175117 Numbers k such that (-1)^k*A008347(k) is a perfect square. - _R. J. Mathar_, Feb 21 2010
%H A175117 Harvey P. Dale, <a href="/A175117/b175117.txt">Table of n, a(n) for n = 1..300</a>
%e A175117 2 - 3 + 5 = 4, thus a(1)=3;
%e A175117 2 - 3 + 5 - 7 + 11 - 13 + 17 - 19 + 23 = 16, thus a(2)=9.
%t A175117 Flatten[Position[Accumulate[Times@@@Partition[Riffle[Prime[Range[ 20000]], {1,-1},{2,-1,2}],2]],_?(IntegerQ[Sqrt[#]]&)]] (* _Harvey P. Dale_, Mar 19 2013 *)
%Y A175117 Cf. A000040.
%K A175117 easy,nonn
%O A175117 1,1
%A A175117 _Ctibor O. Zizka_, Feb 14 2010
%E A175117 Six more terms from _R. J. Mathar_, Feb 21 2010