%I #8 Mar 07 2015 06:12:06
%S 1,2,4,9,24,63,177,514,1527,4625,14230,44358,139779,444558,1425151,
%T 4600339,14939849,48778197,160019885,527200711,1743607826,5786756470,
%U 19266336882,64331275266,215377539119,722840378042,2431459847364
%N Another approximation to A000084(n).
%C A more accurate version of A001573.
%D J. Riordan and C. E. Shannon, The number of two-terminal series-parallel networks, J. Math. Phys., 21 (1942), 83-93. Reprinted in Claude Elwood Shannon: Collected Papers, edited by N. J. A. Sloane and A. D. Wyner, IEEE Press, NY, 1993, pp. 560-570.
%F a(n) is the nearest integer to u(n) (with round(0.5)=0), where u(n) has o.g.f.: U(x) = (5 - 3*x - 2*x^2 - sqrt(9 - 30*x - 11*x^2 + 12*x^3 + 4*x^4))/2. [Riordan & Shannon]
%Y Cf. A000084, A058585.
%K nonn
%O 1,2
%A Pab Ter (pabrlos(AT)yahoo.com), May 11 2004