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Circular map (Posted on 2006-11-15) Difficulty: 5 of 5
Let f be a one-to-one correspondence of the points in a plane. Prove or disprove the following statement:

"If f maps circles to circles, then it maps straight lines to straight lines."

  Submitted by JLo    
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Solution: (Hide)
The statement is true. We prove it in three small bits. For an alternative proof, see posts by Tristan and Joel.

1. If suffices to show that f maps collinear points to collinear points.
Proof: If l is a line then f(l) is at least a sub set of a certain line l'. If there were a point P' on l' such that P:=f-1(P') is outside l, then pick points A and B on l and a circle through A, B and P (There is always a circle through three non-collinear points). But then f(A), f(B) and f(P)=P' lie on a circle, which contradicts that P' is on l'. Therefore f(l)=l'

2. f-1 maps collinear points to collinear points.
Proof: Otherwise there are non-collinear points A, B, C such that A'=f(A), B'=f(B) and C'=f(C) are collinear. But A', B' and C' must lie on a circle, because A, B and C do.

3. f maps collinear points to collinear points.
Proof: Consider three collinear points A, B and P, where P is in between A and B. Let C be the circle through A and P with diameter AP and D the circle through B and P with diameter BP. Denote the images under f by A', B', P', C' and D'. The line through A' and P' intersects D' in a point S'. By statement 2 we know that S:=f-1(S') is on the line through A and P. S is also on the circle D. Since S is unequal P, we have S=B and therefore S'=B'. Hence B' is on the same line as A' and P'.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Line-ear mapsJLo2006-12-21 13:36:50
re(8): Full ProofJLo2006-12-20 14:30:50
re(7): Full ProofTristan2006-12-19 20:18:21
re(7): Full ProofBractals2006-12-19 00:49:38
re(6): Full ProofJoel2006-12-18 22:44:05
re(5): Full ProofBractals2006-12-18 12:53:22
re(4): Full ProofJoel2006-12-18 00:21:46
re(3): Full ProofBractals2006-12-17 15:35:08
re(2): Full ProofTristan2006-12-17 00:20:28
re: Full ProofBractals2006-12-15 18:15:42
SolutionFull ProofTristan2006-12-15 14:56:08
Some ThoughtsFamily of CirclesBractals2006-11-24 10:53:29
re(4): Not a proof, but ...Steve Herman2006-11-24 09:08:14
re(3): Not a proof, but ...Tristan2006-11-24 02:07:22
re(3): Sticking by my gunsJoel2006-11-24 01:30:34
re(2): Not a proof, but ...Steve Herman2006-11-23 09:02:55
re: Not a proof, but ...Tristan2006-11-18 14:49:20
Some ThoughtsNot a proof, but ...Steve Herman2006-11-18 08:54:41
re(2): ProofTristan2006-11-17 15:34:33
re: ProofJLo2006-11-17 12:52:15
re: Question for JLoJLo2006-11-17 12:46:52
SolutionProofTristan2006-11-16 20:44:19
QuestionQuestion for JLoBractals2006-11-16 17:37:48
re(2): Sticking by my gunshenry2006-11-16 16:39:41
re: Sticking by my gunsJLo2006-11-16 12:35:32
re: A thoughtCharlie2006-11-16 10:42:11
A thoughtBernie Hunt2006-11-16 10:13:08
Some ThoughtsA thoughtBractals2006-11-15 18:48:19
Some ThoughtsSticking by my gunsFederico Kereki2006-11-15 18:02:49
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