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%I #4 Mar 30 2012 16:47:42
%S 1,2,2,6,2,18,2,46,14,126,2,454,2,1266,134,4222,2,14202,2,46406,1274,
%T 155502,2,531790,122,1792810,13802,6126334,2,21032794,2,72121854,
%U 155510,248396798,1382,857944150,2,2964896882,1792818,10269600622
%N Numbers d_n used in recurrence for series-parallel numbers.
%D J. Riordan, An Introduction to Combinatorial Analysis, foot of p. 141.
%F G.f.: S'*(1-S)/(1+S), where S = g.f. for A000084.
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_.