Whitney, H.: Non-separable and planar graphs. Trans. AMS 34 (1932), 339–362H. Whitney. Non-separable and planar graphs. Trans. Amer. Math. Soc. 34(2):339-362, 1932, doi:10.2307/1989545.H. Whitney. Non-separable and planar graphs. Transactions of the American Mathematical Society, ...