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%I #5 May 02 2013 00:19:42
%S 1,1,2,4,9,23,63,177,514,1526,4624,14230,44357,139779,444557,1425151,
%T 4600338,14939849,48778197,160019884,527200711,1743607825,5786756469,
%U 19266336882,64331275266,215377539119,722840378041,2431459847363
%N An approximation to A000084(n).
%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) = floor(u(n)) where u(n) has g.f. (1/2)*(5-3*x-2*x^2-sqrt(9-30*x-11*x^2+12*x^3+4*x^4)).
%Y Cf. A000084, A001573, A093156.
%K nonn
%O 0,3
%A _N. J. A. Sloane_, Dec 26 2000
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