17.3: Bolyai's construction
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Assume we need to construct a line thru P asymptotically parallel to the given line ℓ in the h-plane.
If A and B are ideal points of ℓ in the projective model, then we could simply draw the Euclidean line (PA). However the ideal points do not lie in the h-plane; therefore there is no way to use them in the construction.
In the following construction we assume that you know a compass-and-ruler construction of the perpendicular line; see Exercise 5.22.
- Drop a perpendicular from P to ℓ; denote it by m. Let Q be the foot point of P on ℓ.
- Erect a perpendicular from P to m; denote it by n.
- Mark by R a point on ℓ distinct from Q.
- Drop a perpendicular from R to n; denote it by k.
- Draw the circle Γ with center P and the radius QR. Mark by T a point of intersection of Γ with k.
- The line (PT)h is asymptotically parallel to ℓ.
Explain what happens if one performs the Bolyai construction in the Euclidean plane.
- Answer
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To prove that Bolyai’s construction gives the asymptotically parallel line in the h-plane, it is sufficient to show the following:
Assume P, Q, R, S, T be points in h-plane such that
- S∈(RT)h,
- (PQ)h⊥(QR)h,
- (PS)h⊥(PQ)h,
- (RT)h⊥(PS)h and
- (PT)h and (QR)h are asymptotically parallel.
Then QRh=PTh.
- Proof
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We will use the projective model. Without loss of generality, we may assume that P is the center of the absolute. As it was noted on page , in this case the corresponding Euclidean lines are also perpendicular; that is, (PQ)⊥(QR), (PS)⊥(PQ), and (RT)⊥(PS).
Let A be the common ideal point of (QR)h and (PT)h. Let B and C denote the remaining ideal points of (QR)h and (PT)h respectively.
Note that the Euclidean lines (PQ), (TR), and (CB) are parallel.
Therefore,
In particular,
ACAB=ATAR=APAQ.
It follows that
In particular, AT⋅CPTC⋅PA=AR⋅BQRB⋅QA. Applying the formula for h-distance 17.2.1, we get that QRh=PTh.