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 A028400 #35 Aug 02 2025 17:04:09
%S A028400 4,9,25,81,289,1089,4225,16641,66049,263169,1050625,4198401,16785409,
%T A028400 67125249,268468225,1073807361,4295098369,17180131329,68720001025,
%U A028400 274878955521,1099513724929,4398050705409,17592194433025
%N A028400 a(n) = (2^n + 1)^2.
%H A028400 H. Bottomley, <a href="/A060919/a060919.gif">Illustration of initial terms</a>
%H A028400 I. Strazdins, <a href="https://doi.org/10.1023/A:1005769927571">Universal affine classification of Boolean functions</a>, Acta Applic. Math. 46 (1997), 147-167.
%H A028400 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (7,-14,8).
%F A028400 a(n) = A000051(n)^2. - _R. J. Mathar_, Nov 27 2015
%F A028400 G.f.: ( -4+19*x-18*x^2 ) / ( (x-1)*(2*x-1)*(4*x-1) ). - _R. J. Mathar_, Nov 27 2015
%t A028400 LinearRecurrence[{7,-14,8},{4,9,25},30] (* _Harvey P. Dale_, Aug 02 2025 *)
%o A028400 (PARI) a(n)=(2^n + 1)^2
%K A028400 nonn,easy
%O A028400 0,1
%A A028400 _N. J. A. Sloane_