Enumeration and limit laws of series-parallel graphs

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We show that the number gₙ of labelled series-parallel graphs on n vertices is asymptotically gₙ g· n⁻⁵/² γⁿ n!, where γ and g are explicit computable constants. We show that the number of edges in random series-parallel graphs is asymptotically normal with linear mean and variance, and that the number of edges is sharply concentrated around its expected value. Similar results are proved for labelled outerplanar graphs and for graphs not containing K₂,₃ as a minor.