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Two distinct uniform points on a sphere from one uniform distribution via a regu

Two distinct uniform points on a sphere from one uniform distribution via a regu
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2021-01-22 02:43
VectorSolid GeometryProbability & StatisticsGeometric Construction
This example demonstrates how to generate two different points on a sphere that are uniformly distributed, using only a single underlying uniform random source, with a regular tetrahedron (with parameter V=4) providing the construction that controls how the two points differ while preserving uniformity on the sphere. Key concepts include spherical geometry, vectors, rotation/angle relations, and probability-style uniform sampling constraints on a curved surface. The construction uses GeoGebra commands and objects such as Sphere and Circle to define the spherical setting and auxiliary geometry, PointIn to test membership in relevant regions, Vector and Angle to build directional relationships, RotateText and PolyLine/Polygon to visualize transformations and the geometric structure, and If together with Max/Min and Length to enforce selection/feasibility conditions for the second point. It also employs Textfield/TableText to display computed parameters (e.g., angles/thresholds and bounds like Zmin/Zmax, fmin3D/fmax3D) and CurveCartesian to plot verification curves associated with the constraints. Suitable for high school through early university learners studying geometry, computational modeling, and probability-inspired sampling on manifolds, and for GeoGebra-based demonstrations of spherical uniformity checks.

GeoGebra Commands & Parameters

  1. Polygon({((-π), ((-π)) / 2), ((-π), π / 2), (π, π / 2), (π, ((-π)) / 2)}) → q_2
  2. PointIn(q_2) → Atest_2
  3. Function(f01, (-π), π, ((-π)) / 2, π / 2) → f0
  4. Zip((x(a), y(a), f0(a)), a, Zmin_2) → fmin3D
  5. Zip((x(a), y(a), f0(a)), a, Zmax_2) → fmax3D
  6. Vector((1.3 * Z3), (1.5 * Z3)) → AVZ3r
  7. Vector((1.3 * Z3), (1.3 * Z3) + UnitVector[Sum[Zip[UnitVector[a - Z3], a, lP]]]) → AVZ3
  8. Vector((1.3 * Z2), (1.5 * Z2)) → AVZ2r
  9. Vector((1.3 * Z2), (1.3 * Z2) + UnitVector[Sum[Zip[UnitVector[a - Z2], a, lP]]]) → AVZ2
  10. Vector((1.3 * Z1), (1.5 * Z1)) → AVZ1r
  11. Vector((1.3 * Z1), (1.3 * Z1) + UnitVector[Sum[Zip[UnitVector[a - Z1], a, lP]]]) → AVZ1
  12. Vector((1.3 * Atest_R), (1.3 * Atest_R) + UnitVector[Sum[Zip[UnitVector[a - Atest_R], a, lP]]]) → AVtest
  13. Vector((1.3 * Atest_R), (1.5 * Atest_R)) → AVtestr
  14. Sphere(O, R) → sphera
  15. Circle(O, R, xOyPlane) → c
  16. PointIn(q_2) → RZ1_2
  17. PointIn(q_2) → RZ2_2
  18. Angle(AVtest, AVtestr) → αA
  19. Angle(AVZ1, AVZ1r) → αZ1
  20. Angle(AVZ2, AVZ2r) → αZ2
  21. Zip(Segment[Atest_R, p], p, lP) → seg
  22. Sum(seg) → sum_seg
  23. PolyLine({((-π), ((-π)) / 2), ((-π), π / 2), (π, π / 2), (π, ((-π)) / 2), ((-π), ((-π)) / 2)}) → q1
  24. Flatten(Sequence[Sequence[((-π) + ((2 * π / na) * i), ((-π)) / 2 + (π / na * j)), j, 1, na], i, 1, na]) → ltest_2
  25. Zip((R; x(a); y(a)), a, ltest_2) → ltest_3
  26. Length(lZmax') → lälZmax'
  27. Textfield(n) → InputBox11
  28. alfa(R, lP, lZsad') → αsad
  29. If(αZ1 > 100°, 180° - αZ1, αZ1) → αZ1'
  30. Textfield(m_1) → InputBox12
  31. Textfield(t) → InputBox13
  32. Textfield(m_2) → InputBox14
  33. PointIn(q_2) → RZ3_2
  34. Sum(Zip[1 / Length[p - Z3], p, lP]) → ZahZ3
  35. Angle(AVZ3, AVZ3r) → αZ3
  36. Textfield(R) → InputBox1
  37. Textfield(na) → InputBox2
  38. Max(f(Zmax_2)) → max_f
  39. Min(f(Zmin_2)) → min_f
  40. Zip(Text["d_{" + (a) + "}", (Atest_R + Element[lP, a]) / 2], a, 1…n) → lPtxt
  41. Zip(Segment[F_o, p], p, lP) → seg0
  42. Sum(seg0) → sum_seg0
  43. RotateText(txt, 90°) → txt1
  44. Sum(lZ3sad) → sumlZ3sad
  45. Textfield(Points3D) → InputBox7
  46. Textfield(lZsad') → InputBox17
  47. Textfield(lZmin') → InputBox16
  48. Textfield(lZmax') → InputBox15
  49. Textfield(jmin) → InputBox18
  50. Textfield(jmax) → InputBox19
  51. Textfield(jsad) → InputBox20
  52. Zip(Text["" + (b), a - (0.1, 0.1)], a, Zmax_2, b, 1…Length[Zmax_2]) → l2maxtxt
  53. Zip(Text["" + (b), a - (0.1, 0.1)], a, Zmin_2, b, 1…Length[Zmin_2]) → l2mintxt
  54. Zip(Text["" + (b), a - (0.1, 0.1)], a, Zsad_2, b, 1…Length[Zsad_2]) → l2sadtxt
  55. Zip(Vector[r1, (1.153846154 * r1)], r1, (1.3 * Zip[(R * UnitVector[a]), a, lZmax'])) → lVmaxr
  56. Zip(Vector[r1, (1.153846154 * r1)], r1, (1.3 * Zip[(R * UnitVector[a]), a, lZmin'])) → lVminr
  57. Zip(Vector[r1, (1.153846154 * r1)], r1, (1.3 * Zip[(R * UnitVector[a]), a, lZsad'])) → lVsadr
  58. alfa(R, lP, lZmax') → αmax
  59. alfa(R, lP, lZmin') → αmin
  60. arrow(R, lP, lZmax') → lVmax
  61. arrow(R, lP, lZmin') → lVmin
  62. arrow(R, lP, lZsad') → lVsad
  63. CurveCartesian((-π) + ((2 * π) * a), ((-π)) / 2, f0((-π) + ((2 * π) * a), ((-π)) / 2), a, 0, 1) → par1
  64. CurveCartesian((-π) + ((2 * π) * a), π / 2, f0((-π) + ((2 * π) * a), π / 2), a, 0, 1) → par2
  65. CurveCartesian((-π), ((-π)) / 2 + (π * a), f0((-π), ((-π)) / 2 + (π * a)), a, 0, 1) → par3
  66. CurveCartesian(π, ((-π)) / 2 + (π * a), f0(π, ((-π)) / 2 + (π * a)), a, 0, 1) → par4
  67. Polygon({((-π), ((-π)) / 2), ((-π), π / 2), (π, π / 2), (π, ((-π)) / 2)}) → q_3
  68. Textfield(achs) → InputBox3
  69. Circle(Atest_2, 0.2) → ci
  70. TableText(Append["i", Sequence[Length[lZsad']]], Append["\frac{N°}{levels} ", msad], Append["P_{sad }", lZsad'], Append["(f'x,f'y)", lZf'xysad], Append[" Δα≟ π ", αsad], "v") → text2c
  71. TableText(Append["i", Sequence[Length[αmax]]], Append["\frac{N°}{levels} ", mmax], Append["P_{Max }", lZmax'], Append["(f'x,f'y)", lZf'xymax], Append[" Δα≟ π ", αmax], "v") → text2a
  72. TableText(Append["i", Sequence[Length[αmin]]], Append["\frac{N°}{levels} ", mmin], Append["P_{Min }", lZmin'], Append["(f'x,f'y)", lZf'xymin], Append[" Δα≟ π ", αmin], "v") → text2b
  73. Zip((x(a), y(a), f0(a)), a, Zsad_2) → fsad3D
  74. Zip(Atest_2 + (r_c; ((2 * π / n_c) * k)), k, 0…(n_c - 1)) → l0r
  75. Sum(Sequence[If[sum00_r(i) ≠ sum00_r(i + 1), 1, 0], i, 1, Length[sum00_r] - 1]) → sum01_r
  76. Sum(Unique[sgn(f(Atest_2) - f(l0r))]) → sum_r

AI Drawing Prompt Reference

Draw a 3D sphere with a common center and radius, then visualize its parameter-domain boundaries using planar closed polylines. In the base plane (the plane containing the sphere’s defining circle), plot a quadrilateral outline corresponding to fixed angle ranges, and also draw a second quadrilateral outline with subdivided boundary points for verification. Choose a reference point whose direction lies inside the first closed region, then construct another direction on the sphere by enforcing the relation between the reference vector and a transformed/rotated direction so that the angle between them can be measured. Include angle annotations for the constructed directional pairs and indicate the constrained feasible regions relative to the drawn boundaries.
The prompt above was compiled from the original drawing commands and is for reference only.