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 A115065 #20 Jul 07 2023 14:45:49
%S A115065 1,2,4,6,10,14,20,26,33,40,49,58,69,80,93,106,120,134,150,166,184,202,
%T A115065 222,242,263,284,307,330,355,380,406,432,460,488,518,548,580,612,645,
%U A115065 678,713,748,785,822,861,900,940,980,1022
%N A115065 Number of points with integer coordinates inside the equilateral triangle with base [0,n].
%C A115065 If one replaces the equilateral triangle by a rectangle isoscele triangle with hypotenuse [0,n], one gets A087811. - _Michel Marcus_, Aug 05 2013
%e A115065 For n=0, the triangle is degenerate and there is 1 point (0,0).
%e A115065 For n=1, there are 2 points (0,0) and (0,1).
%o A115065 (PARI) a(n) = {nb = 0; for (x=0, n, for (y=0, n, if ((x < n/2) && (y <= x*sqrt(3)), nb++); if ((x >= n/2) && (y + x*sqrt(3)) <= n*sqrt(3), nb++););); nb;} \\ _Michel Marcus_, Aug 05 2013
%K A115065 nonn
%O A115065 0,2
%A A115065 Neven Juric (neven.juric(AT)apis-it.hr), Jun 20 2006
%E A115065 Offset set to 0 and a(0) prepended by _Michel Marcus_, Aug 05 2013