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 A227805 #54 Aug 27 2022 03:16:17
%S A227805 2,6,0,8,-2,10,-4,12,-6,14,-8,16,-10,18,-12,20,-14,22,-16,24,-18,26,
%T A227805 -20,28,-22,30,-24,32,-26,34,-28,36,-30,38,-32,40,-34,42,-36,44,-38,
%U A227805 46,-40,48,-42,50,-44,52,-46,54,-48,56,-50,58,-52,60,-54,62,-56,64
%N A227805 Sum of even numbers starting at 2, alternating signs.
%C A227805 Twice A195084. - _Joerg Arndt_, Aug 14 2013
%C A227805 The 1st, 3rd, 5th, 7th, ... "terms" increase by 2, the 2nd, 4th, 6th, 8th, ... decrease by 2. Also the difference between the terms goes up by 2 each time. For example, the difference between 6 and 0 = 6, difference between 0 and 8 = 8, difference between 8 and -2 = 10, and so on.
%C A227805 Also the sequence seems to "mirror" a few terms before the 6, i.e.: -4, 10, -2, 8, 0, 6, 2, 4,[mirror],4, 2, 6, 0, 8, -2
%H A227805 Vincenzo Librandi, <a href="/A227805/b227805.txt">Table of n, a(n) for n = 1..1000</a>
%H A227805 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,1,1).
%F A227805 a(n) = 2 * A195084(n). - _Joerg Arndt_, Aug 14 2013
%F A227805 G.f.: 2*x*(1+4*x+2*x^2)/((1-x)*(1+x)^2). - _Joerg Arndt_, Aug 14 2013
%e A227805 Using the sequence 2+4-6+8-10+12-14-16 gives (2+4)=6, (6-6)=0, (0+8)=8, (8-10)=-2, (-2+12)=10, etc., giving the sequence 6,0,8,-2,10,-4,12,-6,14,-8.
%t A227805 nn = 100; s = 2 Range[2, nn]*Table[(-1)^i, {i, 2, nn}]; s = Join[{2}, s]; Accumulate[s] (* _T. D. Noe_, Aug 13 2013 *)
%Y A227805 Cf. A195084.
%K A227805 sign,less
%O A227805 1,1
%A A227805 _D.Wilde_, Aug 02 2013