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 A297189 #17 Oct 13 2019 12:08:20 %S A297189 0,1,3,6,21,39,141,258,939,1713,6243,11382,41493,75639,275757,502674, %T A297189 1832619,3340641,12179139,22201062,80939541,147542727,537904077, %U A297189 980532258,3574776747,6516373521,23757077283,43306197846,157883627541,287802221079 %N A297189 Expansion of (x + 3*x^2 - 2*x^3 - 3*x^4)/(1 - 8*x^2 + 9*x^4). %C A297189 Related to a tiling of the plane by heptagons. %H A297189 Colin Barker, <a href="/A297189/b297189.txt">Table of n, a(n) for n = 0..1000</a> %H A297189 Peter Steinbach, <a href="http://www.jstor.org/stable/2691048">Golden fields: a case for the heptagon</a>, Math. Mag. Vol. 70, No. 1, Feb. 1997, 22-31. %H A297189 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,8,0,-9). %F A297189 a(n) = 8*a(n-2) - 9*a(n-4). - _Colin Barker_, Jan 05 2018 %F A297189 a(2*n)/a(2*n-1) ~ 2*a(2*n+1)/a(2*n) ~ 1 + sqrt(7). - _Kyle MacLean Smith_, Oct 11 2019 %o A297189 (PARI) concat(0, Vec((x + 3*x^2 - 2*x^3 - 3*x^4)/(1 - 8*x^2 + 9*x^4) + O(x^40))) \\ _Colin Barker_, Jan 05 2018 %K A297189 nonn,easy %O A297189 0,3 %A A297189 _N. J. A. Sloane_, Jan 04 2018, following a suggestion from _Roger L. Bagula_