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 A331755 Number of vertices in a regular drawing of the complete bipartite graph K_{n,n}. 23
 2, 5, 13, 35, 75, 159, 275, 477, 755, 1163, 1659, 2373, 3243, 4429, 5799, 7489, 9467, 11981, 14791, 18275, 22215, 26815, 31847, 37861, 44499, 52213, 60543, 70011, 80347, 92263, 105003, 119557, 135327, 152773, 171275, 191721, 213547, 237953 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS N. J. A. Sloane, Table of n, a(n) for n = 1..1000 Lars Blomberg, Scott R. Shannon, N. J. A. Sloane, Graphical Enumeration and Stained Glass Windows, 1: Rectangular Grids, (2020). Also arXiv:2009.07918. M. Griffiths, Counting the regions in a regular drawing of K_{n,n}, J. Int. Seq. 13 (2010) # 10.8.5, Lemma 2. S. Legendre, The Number of Crossings in a Regular Drawing of the Complete Bipartite Graph, J. Integer Seqs., Vol. 12, 2009. Scott R. Shannon, Images of vertices for n=2. Scott R. Shannon, Images of vertices for n=3. Scott R. Shannon, Images of vertices for n=4. Scott R. Shannon, Images of vertices for n=5. Scott R. Shannon, Images of vertices for n=6 Scott R. Shannon, Images of vertices for n=7 Scott R. Shannon, Images of vertices for n=8 Scott R. Shannon, Images of vertices for n=9 Scott R. Shannon, Images of vertices for n=10. Scott R. Shannon, Images of vertices for n=12. Scott R. Shannon, Images of vertices for n=15. Eric Weisstein's World of Mathematics, Complete Bipartite Graph FORMULA a(n) = A290132(n) - A290131(n) + 1. a(n) = A159065(n) + 2*n. This is column 1 of A331453. a(n) = (9/(8*Pi^2))*n^4 + O(n^3 log(n)). Asymptotic to (9/(2*Pi^2))*A000537(n-1). [Stéphane Legendre, see A159065.] MAPLE # Maple code from N. J. A. Sloane, Jul 16 2020 V106i := proc(n) local ans, a, b; ans:=0; for a from 1 to n-1 do for b from 1 to n-1 do if igcd(a, b)=1 then ans:=ans + (n-a)*(n-b); fi; od: od: ans; end; # A115004 V106ii := proc(n) local ans, a, b; ans:=0; for a from 1 to floor(n/2) do for b from 1 to floor(n/2) do if igcd(a, b)=1 then ans:=ans + (n-2*a)*(n-2*b); fi; od: od: ans; end; # A331761 A331755 := n -> 2*(n+1) + V106i(n+1) - V106ii(n+1); MATHEMATICA a[n_]:=Module[{x, y, s1=0, s2=0}, For[x=1, x<=n-1, x++, For[y=1, y<=n-1, y++, If[GCD[x, y]==1, s1+=(n-x)*(n-y); If[2*x<=n-1&&2*y<=n-1, s2+=(n-2*x)*(n-2*y)]]]]; s1-s2]; Table[a[n]+ 2 n, {n, 1, 40}] (* Vincenzo Librandi, Feb 04 2020 *) CROSSREFS Cf. A290131 (regions), A290132 (edges), A333274 (polygons per vertex), A333276, A159065. For K_n see A007569, A007678, A135563. Sequence in context: A029885 A114298 A112839 * A137674 A048781 A291242 Adjacent sequences:  A331752 A331753 A331754 * A331756 A331757 A331758 KEYWORD nonn AUTHOR N. J. A. Sloane, Feb 02 2020 STATUS approved

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Last modified June 20 09:57 EDT 2021. Contains 345162 sequences. (Running on oeis4.)