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 A181688 #21 Jul 22 2025 08:42:25
%S A181688 1,1,4,8,23,55,144,360,921,2329,5924,15024,38159,96847,245888,624176,
%T A181688 1584593,4022609,10211940,25924088,65811431,167069767,424126160,
%U A181688 1076693080,2733310377,6938824361,17615009476,44717740000,113521160607,288186606623
%N A181688 Number of maximal self-avoiding walks from NW to SW corners of a 4-by-n grid.
%H A181688 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2, 2, -2, 1).
%F A181688 G.f.: (x^2-x)/(x^4-2*x^3+2*x^2+2*x-1).
%F A181688 a(n) = 2*a(n-1) + 2*a(n-2) - 2*a(n-3) + a(n-4), n > 4.
%e A181688 Illustration of a(1)=a(2)=1:
%e A181688 . .__.
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%e A181688 Illustration of a(3)=4:
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%t A181688 LinearRecurrence[{2, 2, -2, 1}, {1, 1, 4, 8}, 30] (* _T. D. Noe_, Nov 06 2013 *)
%Y A181688 Row 4 of A271592.
%Y A181688 Cf. A000532, A014524, A014523, A181689, A003695, A006864.
%K A181688 nonn,walk
%O A181688 1,3
%A A181688 _Sean A. Irvine_, Nov 17 2010
%E A181688 G.f. formula reverted to the original (correct) value by _Stefan Bühler_, Nov 06 2013