OFFSET
0,3
COMMENTS
We count only configurations where any two circles are either disjoint or meet in two points: We do not allow tangential contacts. A point on a circle must belong to only one or at most two circles: Three circles must not meet at a point. The circles may have different radii.
This is in the affine plane, rather than the projective plane.
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.
Several variations are possible:
- straight lines instead of circles (see A241600).
- straight lines in general position (see A090338).
- curved lines in general position (see A090339).
- allow circles to meet tangentially but without multiple intersection points (begins 1, 5, ...); more terms are needed.
- again use circles, but allow multiple intersection points (also begins 1, 5, ...); more terms are needed.
- use ellipses rather than circles.
- a question from Walter D. Wallis: what if the circles must all have the same radius?
a(1)-a(5) computed by Jon Wild.
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
From Daniel Forgues, Aug 08-09 2015: (Start)
A representation for the diagrams in a250001.jpg (in the same order):
a(1) = 1: {{2}};
a(2) = 3: {{2, 3}, {2, 4}, {4, 6}};
a(3) = 14: {{2, 4, 8}, {2, 3, 6}, {2, 3, 4}, {2, 3, 5}, {4, 6, 5},
{4, 6, 15}, {2, 6, 9}, {4, 6, 12}, {2, 8, 12}, {30, 42, 70},
{?, ?, ?}, {?, ?, ?}, {15, 21, 35}, {?, ?, ?}}.
In lexicographic order:
a(3) = 14: {{2, 3, 4}, {2, 3, 5}, {2, 3, 6}, {2, 4, 8}, {2, 6, 9},
{2, 8, 12}, {4, 6, 5}, {4, 6, 12}, {4, 6, 15}, {15, 21, 35},
{30, 42, 70}, {?, ?, ?}, {?, ?, ?}, {?, ?, ?}}.
The smallest integers greater than 1 are used for the representation:
(p_1)^(a_1)*...*(p_m)^(a_m), where
0 <= a_i <= n, for 1 <= i <= m;
(a_1)+...+(a_m) > 0.
Could the Venn diagram interpretation (of the k-wise, 1 <= k <= n, common divisors of k numbers from each subset) reveal a pattern?
Does this representation work for more complex diagrams? (End)
Once you get to n=5, geometric concerns mean that not all topologically-conceivable arrangements are actually circle-drawable. My program enumerated 16977 conceivable arrangements of 5 pseudo-circles, and Christopher Jones and I together have figured out how to show that 26 of these arrangements are not actually circle-drawable. So it seems that a(5) = 16951. This entry will be updated soon, and there will be a new sequence for the number of topologically-conceivable arrangements. - Jon Wild, Aug 25 2016 [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]
a(n) <= 7*13^(binomial(n,3) + binomial(n,2) + 3n - 1) is a (loose) upper bound, see Reddit link. I believe XkF21WNJ's reply shaves off a factor of 13^3 from this bound for all n > 1. - Linus Hamilton, Apr 14 2019
A good upper bound for a(6) is given in sequence A288559, which counts the arrangements of pseudo-circles, i.e. the topologically conceivable arrangements mentioned above, which are not all necessarily realizable with true circles. The number of arrangements of 6 pseudo-circles was found by Andrii Shportko and Jon Wild to be 17,552,169. - Jon Wild, Jun 03 2025
In A288559, a(5) included 26 non-circularizable pseudocircle arrangements, which generated in turn 132,546 6-pseudocircle descendants. These descendants must be excluded from A250001, which means that a tighter upper bound for A250001(6) is 17,419,623. - Andrii Shportko, Jun 06 2025
REFERENCES
Jon Wild, Posting to Sequence Fans Mailing List, May 15 2014.
LINKS
Giedrius Alkauskas, Triangle unions with maximal number of sides, arXiv:2510.22584 [math.CO], 2025.
Mohammad Arab, Creative proofs in combinations, arXiv:2112.08020 [math.CO], 2021-2022.
Andrew Cook and Luca Viganò, A Game Of Drones: Extending the Dolev-Yao Attacker Model With Movement, Proceedings of the 6th Workshop on Hot Issues in Security Principles and Trust (HotSpot 2020): Affiliated with Euro S&P 2020, IEEE Computer Science Press, Genova, Italy (2020).
David O. H. Cutler, Jonas Karlsson, and Neil J. A. Sloane, Cutting a Pancake with an Exotic Knife, arXiv:2511.15864[math.CO], v3, April 19 2026.
Linus Hamilton, How many ways can circles overlap? - Numberphile, Reddit.
Richard J. Mathar, Topologically Distinct Sets of Non-intersecting Circles in the Plane, arXiv:1603.00077 [math.CO], 2016. [Not directly related, but on a similar subject. - N. J. A. Sloane, Jan 20 2017]
N. J. A. Sloane, Illustration of a(2)=3 and a(3)=14
N. J. A. Sloane, Confessions of a Sequence Addict (AofA2017), slides of invited talk given at AofA 2017, Jun 19 2017, Princeton. Mentions this sequence.
N. J. A. Sloane, "A Handbook of Integer Sequences" Fifty Years Later, arXiv:2301.03149 [math.NT], 2023, pp. 9, 21.
N. J. A. Sloane and Brady Haran, How many ways can circles overlap?, Numberphile video (2019)
Jon Wild, Illustrations of the 173 configurations of four circles [Although we believe that the terms of the sequence are correct, Jon Wild and I suspect that at least one of the drawings in this file is wrong. On the other hand, the svg file in the next link is beieved to be correct. - N. J. A. Sloane, Sep 30 2025]
Jon Wild, Illustrations of the 112 connected configurations of four circles (Computer-generated svg file. To see it, save file, open it with a program - such as Chrome - that can handle svg files.)
EXAMPLE
a(2) = 3, because two circles can either be next to each other, overlap with two intersection points, or one may be located within the other (of larger radius). (As per the first comment, the limiting case where they touch in one point is [somewhat arbitrarily] excluded. This would add two more independent configurations, where one touched the other "from inside" or "from outside".) - M. F. Hasler, May 03 2025
KEYWORD
nonn,more,nice,hard
AUTHOR
Jon Wild, May 16 2014
EXTENSIONS
a(4) is 173, not 168. Corrected by Jon Wild, Aug 08 2015
A duplicate pair of configurations in an older file was spotted by Manfred Scheucher, Aug 13 2016.
STATUS
approved
