%I M3692 #12 Nov 21 2017 20:17:43
%S 4,60,588,4636,31932,200364,1174492,6538492,34965772,181084796,
%T 913687100,4511834156,21880671292,104497300828,492527133804,
%U 2295081478492,10588446843324,48422608206348,219723559153052,990104070700956,4433734940648588
%N Almost-convex polygons of perimeter 2n on square lattice.
%D I. G. Enting, A. J. Guttmann, L. B. Richmond and N. C. Wormald, Enumeration of almost-convex polygons on the square lattice, Random Structures Algorithms 3 (1992), 445-461.
%D K. Y. Lin, Number of almost-convex polygons on the square lattice, J. Phys. A: Math. Gen. 25 (1992), 1835-1842.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%F G.f. 16*x^3*A / ((1-x) * (1-4*x)^(5/2)) + 4*x^3*B / ((1-x) * (1-3*x+x^2) * (1-4*x)^3) where A = 1 - 9*x + 25*x^2 - 23*x^3 + 3*x^4 and B = -4 + 56*x - 300*x^2 + 773*x^3 - 973*x^4 + 535*x^5 - 90*x^6 + 24*x^7 [from Lin]. - _Sean A. Irvine_, Nov 21 2017
%K nonn
%O 6,1
%A _Simon Plouffe_
%E More terms from _Sean A. Irvine_, Nov 21 2017