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Conference rusure::math

Title:Mathematics at DEC
Moderator:RUSURE::EDP
Created:Mon Feb 03 1986
Last Modified:Fri Jun 06 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:2083
Total number of notes:14613

745.0. "Wildly discontinuous functions, pt. 2" by SQM::HALLYB (Like a breath of fresh water...) Thu Jul 30 1987 16:41

    Is there a function from (a1,b1) onto (a2,b2) that is both monotonic 
    and nowhere continuous?  (Here (a,b) represents the open interval 
    between real numbers a and b, a < b).

      John
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745.1TLE::BRETTThu Jul 30 1987 17:3922
    No.
    
    Consider the limit of the following process.
    
    	Define L(0) = a1, H(0) = a2	M(0) = (L(0) + H(0))/2
    
    	Now f(M(0)) is either > (F(L(0)) + F(H(0))/2 or its not.
    
    	If it is greater, then define L(1) = M(0), H(1) = H(0),
    			  else define L(1) = L(0), H(1) = M(0)
    
    and repeat (ie, its basically a bisection approach choosing the
    flatter side of the two sides each time).
    
    Now, what can you say about LIM(M(i))?
    
    You can say that the area �0.5**j*(a2-a1) about it is bounded by
    			      �0.5**(j-1)*(b2-b1)
    
    So the function is continuous at that point.
    
    /Bevin
745.2KIRK::KOLKERConan the LibrarianThu Jul 30 1987 19:1811
    reply .0
    
    A monotone function can only have jump discontinuities, i.e. the
    left hand limit at a point of discontinuity is less than the right
    hand limit at that point (if the function is monotone increasing,
    reverse if not).  Since each jump discontinuity defines an interval
    and each interval contains a rational (rationals are dense in the
    reals) it follows that there can only be a countable number of such
    discontinuities.  This show that the answer to your question is
    no.