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 A138238 #7 Feb 13 2018 12:52:05
%S A138238 0,1,0,9,-16,105,-336,1513,-5712,23529,-92752,373737,-1489488,5968873,
%T A138238 -23853648,95458281,-381745744,1527157737,-6108281424,24433824745,
%U A138238 -97733900880,390938399721,-1563748006480
%N A138238 Alternating sum of the squares of the first n Jacobsthal numbers.
%H A138238 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (-2,9,2,-8).
%F A138238 G.f. : x(1+2x)/((1-x^2)(1+2x-8x^2));
%F A138238 a(n)=4*2^n/9-4(-4)^n/45-(-1)^n/18-3/10;
%F A138238 a(n)=(13-4*J(2n+1))(-1)^n/30+4*J(n)/3-3/10, J(n)=A001045(n);
%F A138238 a(n)=sum{k=0..n (-1)^(k+1)*J(k)^2};
%t A138238 LinearRecurrence[{-2,9,2,-8},{0,1,0,9},30] (* _Harvey P. Dale_, Feb 13 2018 *)
%Y A138238 Cf. A119283.
%K A138238 easy,sign
%O A138238 0,4
%A A138238 _Paul Barry_, Mar 07 2008