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 A051938 #38 Dec 13 2022 02:08:14
%S A051938 3,10,18,27,37,48,60,73,87,102,118,135,153,172,192,213,235,258,282,
%T A051938 307,333,360,388,417,447,478,510,543,577,612,648,685,723,762,802,843,
%U A051938 885,928,972,1017,1063,1110,1158,1207,1257,1308,1360,1413,1467,1522,1578
%N A051938 Truncated triangular numbers: a(n) = n*(n+1)/2 - 18.
%C A051938 If a 3-set Y and a 3-set Z, having one element in common, are subsets of an n-set X then a(n+2) is the number of 3-subsets of X intersecting both Y and Z. - _Milan Janjic_, Oct 03 2007
%H A051938 Colin Barker, <a href="/A051938/b051938.txt">Table of n, a(n) for n = 6..1000</a>
%H A051938 Milan Janjic, <a href="https://pmf.unibl.org/wp-content/uploads/2017/10/enumfor.pdf">Two Enumerative Functions</a>.
%H A051938 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A051938 a(n) = n + a(n-1) (with a(6)=3). - _Vincenzo Librandi_, Aug 06 2010
%F A051938 G.f.: x^6*(3*x^2-x-3) / (x-1)^3. - _Colin Barker_, Mar 18 2015
%F A051938 Sum_{n>=6} 1/a(n) = 4423/6120 + 2*Pi*tan(sqrt(145)*Pi/2)/sqrt(145). - _Amiram Eldar_, Dec 13 2022
%t A051938 Drop[Accumulate[Range[60]]-18,5] (* _Harvey P. Dale_, Dec 08 2017 *)
%o A051938 (PARI) Vec(x^6*(3*x^2-x-3)/(x-1)^3 + O(x^100)) \\ _Colin Barker_, Mar 18 2015
%Y A051938 a(n) = A000217(n) - 18 for n>5.
%Y A051938 Cf. A155212. - _Vincenzo Librandi_, Jan 22 2009
%K A051938 easy,nonn
%O A051938 6,1
%A A051938 Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999