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 A239352 van Heijst's upper bound on the number of squares inscribed by a real algebraic curve in R^2 of degree n, if the number is finite. 2
 0, 0, 1, 12, 48, 130, 285, 546, 952, 1548, 2385, 3520, 5016, 6942, 9373, 12390, 16080, 20536, 25857, 32148, 39520, 48090, 57981, 69322, 82248, 96900, 113425, 131976, 152712, 175798, 201405, 229710, 260896, 295152, 332673, 373660, 418320, 466866, 519517 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS In 1911 Toeplitz conjectured the Square Peg (or Inscribed Square) Problem: Every continuous simple closed curve in the plane contains 4 points that are the vertices of a square. The conjecture is still open. Many special cases have been proved; see Matschke's beautiful 2014 survey. Recently van Heijst proved that any real algebraic curve in R^2 of degree d inscribes either at most (d^4 - 5d^2 + 4d)/4 or infinitely many squares. He conjectured that a generic complex algebraic plane curve inscribes exactly (d^4 - 5d^2 + 4d)/4 squares. REFERENCES Otto Toeplitz, Über einige Aufgaben der Analysis situs, Verhandlungen der Schweizerischen Naturforschenden Gesellschaft in Solothurn, 4 (1911), 197. LINKS G. C. Greubel, Table of n, a(n) for n = 0..2500 Wouter van Heijst, The algebraic square peg problem, arXiv:1403.5979 [math.AG], 2014. Wouter van Heijst, The algebraic square peg problem, Master’s thesis, Aalto University, 2014. Benjamin Matschke, A Survey on the Square Peg Problem, AMS Notices, 61 (2014), 346-352. Benjamin Matschke, Extended Survey on the Square Peg Problem, Max Planck Institute for Mathematics, 2014. Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = (n^4 - 5*n^2 + 4*n)/4 = n*(n - 1)*(n^2 + n - 4)/4, which shows the formula is an integer. G.f.: x^2 * (1 + 7*x - 2*x^2) / (1 - x)^5. - Michael Somos, Mar 21 2014 a(n) = A172225(n)/2. - R. J. Mathar, Jan 09 2018 EXAMPLE A point or a line has no inscribed squares, so a(0) = a(1) = 0. A circle has infinitely many inscribed squares, and an ellipse that is not a circle has exactly one, agreeing with a(2) = 1. G.f. = x^2 + 12*x^3 + 48*x^4 + 130*x^5 + 285*x^6 + 546*x^7 + 952*x^8 + ... MATHEMATICA Table[(n^4 - 5 n^2 + 4 n)/4, {n, 0, 38}] PROG (PARI) for(n=0, 50, print1((n^4 - 5*n^2 + 4*n)/4, ", ")) \\ G. C. Greubel, Aug 07 2018 (MAGMA) [(n^4 - 5*n^2 + 4*n)/4: n in [0..50]]; // G. C. Greubel, Aug 07 2018 CROSSREFS Cf. A088544, A089058, A123673, A123697, A209432, A231739. Sequence in context: A280058 A173548 A006564 * A292022 A265040 A059162 Adjacent sequences:  A239349 A239350 A239351 * A239353 A239354 A239355 KEYWORD nonn,easy AUTHOR Jonathan Sondow, Mar 21 2014 STATUS approved

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Last modified November 21 22:16 EST 2019. Contains 329383 sequences. (Running on oeis4.)