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西爾維斯特四點問題


SylvestersFourPoints

西爾維斯特四點問題詢問的是機率 q(R),即在平面區域 R 中隨機選擇的四個點,其 凸包 是一個 四邊形 (Sylvester 1865)。根據從無限平面中選取點的方法不同,可能存在許多不同的解,促使西爾維斯特得出結論:“這個問題不承認確定的解”(Sylvester 1865;Pfiefer 1989)。

對於從平面 平面 的具有有限 面積 的開凸子集中選取的點,機率由下式給出

 P(R)=1-(4A^__R)/(A(R)),
(1)

其中 A^__R 是區域 R 上三角形的期望面積,A(R) 是區域 R 的面積 (Efron 1965)。請注意,A^__R 只是為適當區域計算的值,例如,圓盤三角形選取三角形三角形選取正方形三角形選取 等,其中 A_R 可以使用 Alikoski 公式精確計算 多邊形三角形選取 的值。

P(R) 可以介於

 2/3<=q(R)<=1-(35)/(12pi^2)
(2)

(0.66666<=q(R)<=0.70448) 之間,具體取決於區域的形狀,正如 Blaschke 首次證明的那樣 (Blaschke 1923, Peyerimhoff 1997)。下表給出了各種簡單平面區域的機率 (Kendall and Moran 1963; Pfiefer 1989; Croft et al. 1991, pp. 54-55; Peyerimhoff 1997)。

RP(R)
三角形2/30.66667
正方形(25)/(36)0.69444
五邊形2/(45)(18-sqrt(5))0.70062
六邊形(683)/(972)0.70267
橢圓, 圓盤1-(35)/(12pi^2)0.70448

西爾維斯特問題可以推廣為詢問機率,即在 單位球 B^n 中隨機選擇的 n+2 個點的 凸包 具有 n+1 個頂點。解由下式給出

 P_n=((n+2)(n+1; 1/2(n+1))^(n+1))/(2^n((n+1)^2; 1/2(n+1)^2))
(3)

(Kingman 1969, Groemer 1973, Peyerimhoff 1997),這對於任何有界的 凸域 K in R^n 來說是最大可能的。前幾個值是

P_1=1
(4)
P_2=(35)/(12pi^2)
(5)
P_3=9/(143)
(6)
P_4=(676039)/(648000pi^4)
(7)
P_5=(20000)/(12964479)
(8)

(OEIS A051050A051051)。

另一個推廣詢問的是機率,即在固定的有界 凸域 K subset R^2 中隨機選擇的 n 個點是凸 n-邊形的頂點。解是

 P_n=(2^n(3n-3)!)/([(n-1)!]^3(2n)!)
(9)

對於三角形域,其前幾個值為 1, 1, 1, 2/3, 11/36, 91/900, 17/675, ... (OEIS A004677A004824),以及

 P_n=[1/(n!)(2n-2; n-1)]^2
(10)

對於平行四邊形域,其前幾個值為 1, 1, 1, 25/36, 49/144, 121/3600, ... (OEIS A004936A005017; Valtr 1996, Peyerimhoff 1997)。

西爾維斯特四點問題與圖的 直線交叉數 有著意想不到的聯絡 (Finch 2003)。


另請參閱

圓盤三角形選取, 六邊形三角形選取, 多邊形三角形選取, 直線交叉數, 正方形三角形選取, 三角形三角形選取

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參考文獻

Alikoski, H. A. "Über das Sylvestersche Vierpunktproblem." Ann. Acad. Sci. Fenn. 51, No. 7, 1-10, 1939.Blaschke, W. "Über affine Geometrie XI: Lösung des 'Vierpunktproblems' von Sylvester aus der Theorie der geometrischen Wahrscheinlichkeiten." Ber. Verh. Sachs. Akad. Wiss. Leipzig Math.-Phys. Kl. 69, 436-453, 1917.Blaschke, W. §24-25 in Vorlesungen über Differentialgeometrie, II. Affine Differentialgeometrie. Berlin: Springer-Verlag, 1923.Croft, H. T.; Falconer, K. J.; and Guy, R. K. "Random Polygons and Polyhedra." §B5 in Unsolved Problems in Geometry. New York: Springer-Verlag, pp. 54-57, 1991.Crofton, M. W. "Probability." Encyclopedia Britannica, Vol. 19, 9th ed. pp. 768-788, 1885.Efron, B. "The Convex Hull of a Random Set of Points." Biometrika 52, 331-343, 1965.Finch, S. R. "Rectilinear Crossing Constant." §8.18 in Mathematical Constants. Cambridge, England: Cambridge University Press, pp. 532-534, 2003.Groemer, H. "On Some Mean Values Associated with a Randomly Selected Simlpex in a Convex Set." Pacific J. Math. 45, 525-533, 1973.Kendall, M. G. and Moran, P. A. P. Geometrical Probability. New York: Hafner, 1963.Kingman, J. F. C. "Random Secants of a Convex Body." J. Appl. Prob. 6, 660-672, 1969.Klee, V. "What is the Expected Volume of a Simplex Whose Vertices are Chosen at Random from a Given Convex Body." Amer. Math. Monthly 76, 286-288, 1969.Peyerimhoff, N. "Areas and Intersections in Convex Domains." Amer. Math. Monthly 104, 697-704, 1997.Pfiefer, R. E. "The Historical Development of J. J. Sylvester's Four Point Problem." Math. Mag. 62, 309-317, 1989.Rottenberg, R. R. "On Finite Sets of Points in P^3." Israel J. Math. 10, 160-171, 1971.Santaló, L. A. Integral Geometry and Geometric Probability. Reading, MA: Addison-Wesley, 1976.Scheinerman, E. and Wilf, H. S. "The Rectilinear Crossing Number of a Complete Graph and Sylvester's 'Four Point' Problem of Geometric Probability." Amer. Math. Monthly 101, 939-943, 1994.Sloane, N. J. A. Sequences A004677, A004824, A004936, A005017, A051050, and A051051 in "The On-Line Encyclopedia of Integer Sequences."Solomon, H. "Crofton's Theorem and Sylvester's Problem in Two and Three Dimensions." Ch. 5 in Geometric Probability. Philadelphia, PA: SIAM, pp. 97-125, 1978.Sylvester, J. J. "Question 1491." The Educational Times (London). April 1864.Sylvester, J. J. "On a Special Class of Questions on the Theory of Probabilities." Birmingham British Assoc. Rept., pp. 8-9, 1865.Valtr, P. "Probability that n Random Points are in a Convex Position." Discrete Comput. Geom. 13, 637-643, 1995.Valtr, P. "The Probability that n Random Points in a Triangle are in Convex Position." Combinatorica 16, 567-573, 1996.Weil, W. and Wieacker, J. "Stochastic Geometry." Ch. 5.2 in Handbook of Convex Geometry (Ed. P. M. Gruber and J. M. Wills). Amsterdam, Netherlands: North-Holland, pp. 1391-1438, 1993.Wilf, H. "On Crossing Numbers, and Some Unsolved Problems." In Combinatorics, Geometry, and Probability: A Tribute to Paul Erdős. Papers from the Conference in Honor of Erdős' 80th Birthday Held at Trinity College, Cambridge, March 1993 (Ed. B. Bollobás and A. Thomason). Cambridge, England: Cambridge University Press, pp. 557-562, 1997.Woolhouse, W. S. B. "Some Additional Observations on the Four-Point Problem." Mathematical Questions, with Their Solutions, from the Educational Times, Vol. 7. London: F. Hodgson and Son, p. 81, 1867.

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西爾維斯特四點問題

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Weisstein, Eric W. "西爾維斯特四點問題。" 來自 Web 資源。 https://mathworld.tw/SylvestersFour-PointProblem.html

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