%I
%S 1,1,3,14,173,16951
%N Number of arrangements of n circles in the affine plane.
%C Two circles are either disjoint or meet in two points. Tangential contacts are not allowed. A point belongs to exactly one or two circles. Three circles may not meet at a point. The circles may have different radii.
%C This is in the affine plane, rather than the projective plane.
%C Two arrangements are considered the same if one can be continuously changed to the other while keeping all circles circular (although the radii may be continuously changed), without changing the multiplicity of intersection points, and without a circle passing through an intersection point. Turning the whole configuration over is allowed.
%C Several variations are possible:
%C  straight lines instead of circles (see A241600).
%C  straight lines in general position (see A090338).
%C  curved lines in general position (see A090339).
%C  allow circles to meet tangentially but without multiple intersection points (begins 1, 5, ...); more terms are needed.
%C  again use circles, but allow multiple intersection points (also begins 1, 5, ...); more terms are needed.
%C  use ellipses rather than circles.
%C  a question from Walter D. Wallis: what if the circles must all have the same radius?
%C a(1)a(4) computed by _Jon Wild_.
%C a(n) >= A000081(n+1)  _Benoit Jubin_, Dec 21 2014. More precisely, there are A000081(n+1) ways to arrange n circles if no two of them meet.  _N. J. A. Sloane_, May 16 2017
%C From _Daniel Forgues_, Aug 0809 2015: (Start)
%C A representation for the diagrams in a250001.jpg (in the same order):
%C a(1) = 1: {{2}};
%C a(2) = 3: {{2, 3}, {2, 4}, {4, 6}};
%C a(3) = 14: {{2, 4, 8}, {2, 3, 6}, {2, 3, 4}, {2, 3, 5}, {4, 6, 5},
%C {4, 6, 15}, {2, 6, 9}, {4, 6, 12}, {2, 8, 12}, {30, 42, 70},
%C {?, ?, ?}, {?, ?, ?}, {15, 21, 35}, {?, ?, ?}}.
%C In lexicographic order:
%C a(3) = 14: {{2, 3, 4}, {2, 3, 5}, {2, 3, 6}, {2, 4, 8}, {2, 6, 9},
%C {2, 8, 12}, {4, 6, 5}, {4, 6, 12}, {4, 6, 15}, {15, 21, 35},
%C {30, 42, 70}, {?, ?, ?}, {?, ?, ?}, {?, ?, ?}}.
%C The smallest integers greater than 1 are used for the representation:
%C (p_1)^(a_1)*...*(p_m)^(a_m), where
%C 0 <= a_i <= n, for 1 <= i <= m;
%C (a_1)+...+(a_m) > 0.
%C Could the Venn diagram interpretation (of the kwise, 1 <= k <= n, common divisors of k numbers from each subset) reveal a pattern?
%C Does this representation work for more complex diagrams? (End)
%C Apart from first term, row sums of triangles A249752 and A252158.  _Omar E. Pol_, Aug 08 2015
%C Comment from _Jon Wild_, Aug 25 2016. Once you get to n=5, geometric concerns mean that not all topologicallyconceivable arrangements are actually circledrawable. My program enumerated 16977 conceivable arrangements of 5 pseudocircles, and Christopher Jones and I together have figured out how to show that 26 of these arrangements are not actually circledrawable. So it seems that a(5) = 16951. This entry will be updated soon, and there will be a new sequence for the number of topologicallyconceivable arrangements. [The counts in this comment were amended by _Jon Wild_ on Aug 30 2016. I apologize for taking so long to make the corrections here.  _N. J. A. Sloane_, Jun 11 2017]
%D Jon Wild, Posting to Sequence Fans Mailing List, May 15 2014.
%H N. J. A. Sloane, <a href="/A250001/a250001_3.pdf">Illustration of a(2)=3 and a(3)=14</a>
%H R. J. Mathar, <a href="http://arxiv.org/abs/1603.00077">Topologically Distinct Sets of Nonintersecting Circles in the Plane</a>, arXiv:1603.00077, 2016. [Not directly related, but on a similar subject.  _N. J. A. Sloane_, Jan 20 2017]
%H N. J. A. Sloane, <a href="/A195264/a195264.pdf">Confessions of a Sequence Addict (AofA2017)</a>, slides of invited talk given at AofA 2017, Jun 19 2017, Princeton. Mentions this sequence.
%H Jon Wild, <a href="/A250001/a250001_4.pdf">Illustrations of the 173 configurations of four circles</a>
%H Jon Wild, <a href="/A250001/a250001.svg">Illustrations of the 112 connected configurations of four circles</a> (Computergenerated svg file. To see it, save file, open it with a program  such as Chrome  that can handle svg files.)
%H Jon Wild, <a href="/A275923/a275923.png">Figure showing relationship between A250001, A275923, A275924, and A288554 for n=3 circles</a>
%Y See A275923 and A275924 for the connected arrangements. See also A288554A288568.
%Y Cf. A241600, A090338, A090339, A048872, A048873, A003036, A132346.
%Y Cf. also A000081, A249752, A252158, A274702, A274776, A274777, A274818, A274819, A285995, A285996, A287149.
%Y Row sums of A261070.
%Y Cf. A132101 (onedimensional analog).
%K nonn,more,nice,hard
%O 0,3
%A _Jon Wild_, May 16 2014
%E a(4) is 173, not 168. Corrected by _Jon Wild_, Aug 08 2015
%E A duplicate pair of configurations in an older file was spotted by _Manfred Scheucher_, Aug 13 2016. The pdf and svg files here are now correct.
