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 A290447 Consider n equally spaced points along a line and join every pair of points by a semicircle above the line; a(n) is the number of intersection points. 33
 0, 0, 0, 1, 5, 15, 35, 70, 124, 200, 300, 445, 627, 875, 1189, 1564, 2006, 2568, 3225, 4035, 4972, 6030, 7250, 8701, 10323, 12156, 14235, 16554, 19124, 22072, 25250, 28863, 32827, 37166, 41949, 47142, 52653, 58794, 65503, 72741, 80437 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Only intersection points above the line are counted. a(n) <= binomial(n,4) (A000332), since that is the number of pairs of intersecting semicircles. See A290461 for the differences. The first time a triple intersection occurs is for n=9. Two fourfold intersections occur for n=13. - Torsten Sillke, Jul 27 2017 If the line is the x-axis and the two semicircles are for (x_1,0),(x_2,0) and (x_3,0),(x_4,0) (with x_1 < x_2, x_3 < x_4, and x_1 < x_3) then they intersect if and only if x_1 < x_3 < x_2 < x_4, and the intersection point has coordinates (x,y) with x=(x_3*x_4 - x_1*x_2) / (x_3 + x_4 - x_1 - x_2) and y^2 = (x_3-x_1)*(x_4-x_1)*(x_2-x_3)*(x_4-x_2) / (x_3 + x_4 - x_1 - x_2)^2.  This allows identification of distinct (and duplicate) intersection points using only rational arithmetic. - David Applegate, Aug 07 2017 Suppose x_i are integers in the range 0 <= x_i < n. Then (x,y) is an intersection point if and only if (n-1-x,y) is an intersection point. Suppose x_4 < n-1. If (x,y) is an intersection point, then (i+x,y) is an intersection point for i = 1,..,n-1-x_4. - Chai Wah Wu, Aug 09 2017 REFERENCES Torsten Sillke, email to N. J. A. Sloane, Jul 27 2017 (giving values for a(1)-a(13)). LINKS David Applegate, Table of n, a(n) for n = 1..500 M. F. Hasler, Illustration for a(9) = 124. (First instance where a triple intersection occurs, whence a(9) < binomial(9,4).) M. F. Hasler, Illustration for a(9) = 124 [Another version, showing baseline] Torsten Sillke, Illustration for a(13) = 627 N. J. A. Sloane, Illustration for a(5) = 5. N. J. A. Sloane, Three (No, 8) Lovely Problems from the OEIS, Experimental Mathematics Seminar, Rutgers University, Oct 05 2017, Part I, Part 2, Slides. (Mentions this sequence) Zahlenjagd, Winter 2010 Problem (asks for a(10)). PROG (PARI) A290447(n, U=[])={for(A=1, n-3, for(C=A+1, n-2, for(B=C+1, n-1, for(D=B+1, n, U=setunion(U, [[(C*D-A*B)/(C+D-A-B), (C-A)*(D-A)*(C-B)*(D-B)/(C+D-A-B)^2]]))))); #U} \\ M. F. Hasler, Aug 07 2017 (Python) from itertools import combinations from fractions import Fraction def A290447(n):     p, p2 = set(), set()     for b, c, d in combinations(range(1, n), 3):         e = b + d - c         f1, f2, g = Fraction(b*d, e), Fraction(b*d*(c-b)*(d-c), e**2), (n-1)*e - 2*b*d         for i in range(n-d):             if 2*i*e < g:                 p2.add((i+f1, f2))             elif 2*i*e == g:                 p.add(f2)             else:                 break     return len(p)+2*len(p2) # Chai Wah Wu, Aug 08 2017 CROSSREFS See A006561 for an analogous problem on a circle. Cf. A290461, A290465, A290726. See A290865, A290866, A290867, A290876 for further properties of these configurations. Sequence in context: A005894 A015622 A292103 * A000750 A289389 A008487 Adjacent sequences:  A290444 A290445 A290446 * A290448 A290449 A290450 KEYWORD nonn AUTHOR N. J. A. Sloane, Aug 05 2017 EXTENSIONS More terms from David Applegate, Aug 07 2017 STATUS approved

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Last modified January 19 03:18 EST 2019. Contains 319282 sequences. (Running on oeis4.)