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 A006402 Number of sensed 2-connected (nonseparable) planar maps with n edges. (Formerly M0812) 5
 1, 2, 3, 6, 16, 42, 151, 596, 2605, 12098, 59166, 297684, 1538590, 8109078, 43476751, 236474942, 1302680941, 7256842362, 40832979283, 231838418310, 1327095781740, 7653155567834, 44434752082990, 259600430870176, 1525366978752096, 9010312253993072, 53485145730576790 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS Some people begin this 2,1,2,3,6,..., others begin it 0,1,2,3,6,.... The dual of a nonseparable map is nonseparable, so the class of all nonseparable planar maps is self-dual. The maps considered here are unrooted and sensed and may include loops and parallel edges. - Andrew Howroyd, Mar 29 2021 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). T. R. S. Walsh, personal communication. LINKS Andrew Howroyd, Table of n, a(n) for n = 2..500 V. A. Liskovets, T. R. S. Walsh, The enumeration of nonisomorphic 2-connected planar maps, Canad. J. Math. 35 (1983), no. 3, 417-435. Timothy R. Walsh, Generating nonisomorphic maps without storing them, SIAM J. Algebraic Discrete Methods 4 (1983), no. 2, 161-178. T. R. S. Walsh, Number of sensed planar maps with n edges and m vertices PROG (PARI) \\ here r(n) is A000139(n-1). r(n)={4*binomial(3*n, n)/(3*(3*n-2)*(3*n-1))} a(n)={(r(n) + sumdiv(n, d, if(d

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