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 A239229 #33 Feb 16 2025 08:33:21
%S A239229 2,0,-2,-4,-6,-8,-10,-12,-14,-16,-18,-20,-22,-24,-26,-28,-30,-32,-34,
%T A239229 -36,-38,-40,-42,-44,-46,-48,-50,-52,-54,-56,-58,-60,-62,-64,-66,-68,
%U A239229 -70,-72,-74,-76,-78,-80,-82,-84,-86,-88,-90,-92,-94,-96,-98,-100,-102
%N A239229 Euler characteristic of n-holed torus: 2 - 2*n.
%D A239229 Jan Gullberg, Mathematics from the Birth of Numbers, W. W. Norton & Co., NY & London, 1997, ยง12.4 Theorems and Formulas (Plane Geometry), p. 450.
%D A239229 James Munkres, Topology, 2nd ed., Pearson, 2000.
%H A239229 Richard Courant and Herbert Robbins, <a href="https://archive.org/details/in.ernet.dli.2015.55043/page/n279/mode/2up">What Is Mathematics?</a>, Oxford, 1941, pp. 258-259.
%H A239229 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/EulerCharacteristic.html">Euler Characteristic</a>.
%H A239229 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A239229 a(n) = 2 - 2*n.
%F A239229 G.f.: 2*(1-2*x)/(1-x)^2. - _Vincenzo Librandi_, Feb 01 2015
%F A239229 a(n) = 2*A022958(n+1). - _R. J. Mathar_, Oct 05 2017
%F A239229 E.g.f.: 2*exp(x)*(1 - x). - _Stefano Spezia_, Sep 10 2022
%p A239229 A239229:=n->2 - 2*n; seq(A239229(n), n=0..100); # _Wesley Ivan Hurt_, Mar 15 2014
%t A239229 Table[2 - 2 n, {n, 0, 100}] (* _Wesley Ivan Hurt_, Mar 15 2014 *)
%t A239229 LinearRecurrence[{2,-1},{2,0},80] (* _Harvey P. Dale_, Jan 31 2015 *)
%t A239229 CoefficientList[Series[2 (1-2 x) / (1 - x)^2, {x, 0, 60}], x] (* _Vincenzo Librandi_, Feb 01 2015 *)
%o A239229 (Magma) [2-2*n: n in [0..60]]; // _Vincenzo Librandi_, Feb 01 2015
%Y A239229 Cf. A010673, A022958.
%K A239229 sign,easy
%O A239229 0,1
%A A239229 _Eric M. Schmidt_, Mar 12 2014