{
  "index": "1977-B-4",
  "type": "GEO",
  "tag": [
    "GEO"
  ],
  "difficulty": "",
  "question": "Problem B-4\nLet \\( C \\) be a continuous closed curve in the plane which does not cross itself and let \\( Q \\) be a point inside \\( C \\). Show that there exist points \\( P_{1} \\) and \\( P_{2} \\) on \\( C \\) such that \\( Q \\) is the midpoint of the line segment \\( P_{1} P_{2} \\).",
  "solution": "B-4.\nWe can assume that \\( Q=O \\), the origin. Let \\( -C \\) be the image of \\( C \\) under the reflection \\( P \\rightarrow-P \\). \\( -C \\) is again a continuous closed curve surrounding \\( O \\) and \\( C \\cap-C \\neq \\varnothing \\) since they have the same diameter and both surround \\( O \\) (hence neither can be exterior to the other). Let \\( P_{1} \\in C \\cap- \\) \\( C \\). Then there exists \\( P_{2} \\in C \\) such that \\( P_{1}=-P_{2} \\). These are the two desired points.",
  "vars": [
    "P_1",
    "P_2",
    "P"
  ],
  "params": [
    "C",
    "Q",
    "O"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "P_1": "pointone",
        "P_2": "pointtwo",
        "P": "pointvar",
        "C": "curvepath",
        "Q": "innerpt",
        "O": "originpt"
      },
      "question": "Problem B-4\nLet \\( curvepath \\) be a continuous closed curve in the plane which does not cross itself and let \\( innerpt \\) be a point inside \\( curvepath \\). Show that there exist points \\( pointone \\) and \\( pointtwo \\) on \\( curvepath \\) such that \\( innerpt \\) is the midpoint of the line segment \\( pointone pointtwo \\).",
      "solution": "B-4.\nWe can assume that \\( innerpt = originpt \\), the origin. Let \\( -curvepath \\) be the image of \\( curvepath \\) under the reflection \\( pointvar \\rightarrow -pointvar \\). \\( -curvepath \\) is again a continuous closed curve surrounding \\( originpt \\) and \\( curvepath \\cap -curvepath \\neq \\varnothing \\) since they have the same diameter and both surround \\( originpt \\) (hence neither can be exterior to the other). Let \\( pointone \\in curvepath \\cap -curvepath \\). Then there exists \\( pointtwo \\in curvepath \\) such that \\( pointone = -pointtwo \\). These are the two desired points."
    },
    "descriptive_long_confusing": {
      "map": {
        "P_1": "marigolds",
        "P_2": "acornleaf",
        "P": "riverstone",
        "C": "sunflower",
        "Q": "raincloud",
        "O": "driftwood"
      },
      "question": "Problem B-4\nLet \\( sunflower \\) be a continuous closed curve in the plane which does not cross itself and let \\( raincloud \\) be a point inside \\( sunflower \\). Show that there exist points \\( marigolds \\) and \\( acornleaf \\) on \\( sunflower \\) such that \\( raincloud \\) is the midpoint of the line segment \\( marigolds acornleaf \\).",
      "solution": "B-4.\nWe can assume that \\( raincloud=driftwood \\), the origin. Let \\( -sunflower \\) be the image of \\( sunflower \\) under the reflection \\( riverstone \\rightarrow -riverstone \\). \\( -sunflower \\) is again a continuous closed curve surrounding \\( driftwood \\) and \\( sunflower \\cap -sunflower \\neq \\varnothing \\) since they have the same diameter and both surround \\( driftwood \\) (hence neither can be exterior to the other). Let \\( marigolds \\in sunflower \\cap - \\) \\( sunflower \\). Then there exists \\( acornleaf \\in sunflower \\) such that \\( marigolds = -acornleaf \\). These are the two desired points."
    },
    "descriptive_long_misleading": {
      "map": {
        "P_1": "voidpointone",
        "P_2": "voidpointtwo",
        "P": "voidpoint",
        "C": "straightness",
        "Q": "exteriority",
        "O": "nonorigin"
      },
      "question": "Problem B-4\nLet \\( straightness \\) be a continuous closed curve in the plane which does not cross itself and let \\( exteriority \\) be a point inside \\( straightness \\). Show that there exist points \\( voidpointone \\) and \\( voidpointtwo \\) on \\( straightness \\) such that \\( exteriority \\) is the midpoint of the line segment \\( voidpointone voidpointtwo \\).",
      "solution": "B-4.\nWe can assume that \\( exteriority = nonorigin \\), the origin. Let \\( -straightness \\) be the image of \\( straightness \\) under the reflection \\( voidpoint \\rightarrow -voidpoint \\). \\( -straightness \\) is again a continuous closed curve surrounding \\( nonorigin \\) and \\( straightness \\cap -straightness \\neq \\varnothing \\) since they have the same diameter and both surround \\( nonorigin \\) (hence neither can be exterior to the other). Let \\( voidpointone \\in straightness \\cap - \\) \\( straightness \\). Then there exists \\( voidpointtwo \\in straightness \\) such that \\( voidpointone = -voidpointtwo \\). These are the two desired points."
    },
    "garbled_string": {
      "map": {
        "P_1": "cqbvldua",
        "P_2": "htrmskeg",
        "P": "fqpdniev",
        "C": "xwzjoguv",
        "Q": "nlsrdvha",
        "O": "kdwyrmte"
      },
      "question": "Problem B-4\nLet \\( xwzjoguv \\) be a continuous closed curve in the plane which does not cross itself and let \\( nlsrdvha \\) be a point inside \\( xwzjoguv \\). Show that there exist points \\( cqbvldua \\) and \\( htrmskeg \\) on \\( xwzjoguv \\) such that \\( nlsrdvha \\) is the midpoint of the line segment \\( cqbvldua htrmskeg \\).",
      "solution": "B-4.\nWe can assume that \\( nlsrdvha=kdwyrmte \\), the origin. Let \\( -xwzjoguv \\) be the image of \\( xwzjoguv \\) under the reflection \\( fqpdniev \\rightarrow-fqpdniev \\). \\( -xwzjoguv \\) is again a continuous closed curve surrounding \\( kdwyrmte \\) and \\( xwzjoguv \\cap-xwzjoguv \\neq \\varnothing \\) since they have the same diameter and both surround \\( kdwyrmte \\) (hence neither can be exterior to the other). Let \\( cqbvldua \\in xwzjoguv \\cap- \\) \\( xwzjoguv \\). Then there exists \\( htrmskeg \\in xwzjoguv \\) such that \\( cqbvldua=-htrmskeg \\). These are the two desired points."
    },
    "kernel_variant": {
      "question": "Let Γ be a simple (i.e. non-self-intersecting) closed rectifiable curve in the plane and let Q be a point strictly inside Γ.  Prove that there exist two distinct points A,B∈Γ such that Q is the midpoint of the segment AB (equivalently, \\(\\overrightarrow{QA}=\\overrightarrow{QB}\\)).",
      "solution": "Step 1.  Translate the configuration so that Q is the origin.\nChoose Cartesian coordinates with Q placed at O=(0,0).  In these coordinates the original curve is still denoted by Γ; it is a simple closed rectifiable curve surrounding O.\n\nStep 2.  Reflect the curve through the origin.\nDefine −Γ := { −P : P∈Γ }.  The map P↦−P is an isometry, so −Γ is congruent to Γ and, because Γ surrounds the origin, −Γ also surrounds O.  In particular, Γ and −Γ are two simple closed curves, each winding once around O.\n\nStep 3.  Reduce the problem to showing that Γ and −Γ intersect.\nIf Γ∩(−Γ) contains two antipodal points P and −P then O is the midpoint of the segment P(−P) and we are done.  Hence it suffices to prove\n            Γ∩(−Γ) ≠ ∅.     (1)\nWe prove (1) by contradiction.\n\nStep 4.  Suppose Γ∩(−Γ)=∅ and derive a contradiction.\nBecause Γ is a Jordan curve, the plane \\(\\mathbb R^{2}\\)\\Γ consists of exactly two connected components: the bounded Jordan region int(Γ) and the unbounded exterior.  Since −Γ also surrounds O, it cannot be contained in the unbounded component; therefore the disjointness hypothesis implies\n            −Γ ⊂ int(Γ).      (2)\n\nStep 5.  A geometric lemma on diameters.\nLemma.  Let K and L be non-empty compact subsets of \\(\\mathbb R^{2}\\) such that K ⊂ int(L).  Then diam(K) < diam(L).\n\nProof.  Because K is strictly contained in int(L), the positive distance\n            δ := dist(K,∂L) > 0.\nChoose a pair of points a,b∈K such that |ab| = diam(K).  Let v = (b−a)/|b−a|.  The points a′ := a−(δ/2)v and b′ := b+(δ/2)v still lie in int(L) (they are δ/2 away from K and hence at least δ/2 away from ∂L).  Consequently a′,b′ ∈ L and\n            |a′b′| = |ab| + δ = diam(K) + δ.\nTherefore diam(L) ≥ |a′b′| > diam(K), proving the lemma.  □\n\nStep 6.  Equality of the diameters of Γ and its Jordan region.\nDenote by Δ := \\overline{int(Γ)} the closed Jordan region bounded by Γ.  Because Γ ⊂ Δ, diam(Δ) ≥ diam(Γ).  We show equality.\nLet X,Y∈Δ realise diam(Δ), i.e. |XY| = diam(Δ).  If both X and Y were interior points of Δ, we could move them slightly along the line XY in opposite directions while remaining inside Δ and thereby increase their distance, contradicting maximality.  Hence at least one of X or Y lies on the boundary ∂Δ = Γ.  Repeating the same argument for the second point shows that both X and Y can be chosen on Γ.  Thus diam(Δ) = diam(Γ).     (3)\n\nStep 7.  Apply the lemma.\nUnder the assumption (2) we have −Γ ⊂ int(Γ) ⊂ int(Δ), so the lemma with K = −Γ and L = Δ gives\n            diam(−Γ) < diam(Δ).\nUsing (3) and the fact that reflection is an isometry (so diam(−Γ)=diam(Γ)), we obtain\n            diam(Γ) < diam(Γ),\na contradiction.  Therefore Γ∩(−Γ) ≠ ∅, establishing (1).\n\nStep 8.  Construct the required points.\nPick any point P ∈ Γ∩(−Γ).  Because P belongs to −Γ, there exists a point P′∈Γ with P′ = −P.  The points are distinct (P=−P would give P=O, which lies inside, not on Γ).  Finally,\n            O = (P + P′)/2,\nso the origin is the midpoint of PP′.  Taking A := P and B := P′ yields the desired points on Γ.\n\nHence for every simple closed rectifiable curve Γ and every interior point Q there exist two distinct points A,B∈Γ such that Q is the midpoint of AB.",
      "_meta": {
        "core_steps": [
          "Translate coordinates so that the given interior point Q becomes the origin O.",
          "Apply the point-reflection P ↦ –P to C, obtaining the congruent curve –C.",
          "Use the Jordan-curve separation fact: two congruent simple closed curves that both enclose O cannot be disjoint, hence C ∩ –C ≠ ∅.",
          "Choose P₁ in the intersection; then its antipode P₂ = –P₁ also lies on C.",
          "The origin (≡ Q) is the midpoint of the segment P₁P₂."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "The reference point to which the figure is translated before reflecting.",
            "original": "the origin (0,0)"
          },
          "slot2": {
            "description": "The specific ‘size’ descriptor used to rule out one curve lying strictly inside the other (any equal invariant would work).",
            "original": "diameter"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}