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<h3 class="section">2.9 Function polyfitinf for polynomial fitting.</h3>

<p><a name="index-polyfitinf-53"></a>

<h4 class="subheading">Helptext:</h4>

<p><a name="XREFpolyfitinf"></a>
<pre class="verbatim">function [A,REF,HMAX,H,R,EQUAL] = polyfitinf(M,N,K,X,Y,EPSH,MAXIT,REF0)

  Best polynomial approximation in discrete uniform norm

  INPUT VARIABLES:

  M       : degree of the fitting polynomial
  N       : number of data points
  X(N)    : x-coordinates of data points
  Y(N)    : y-coordinates of data points
  K       : character of the polynomial:
                  K = 0 : mixed parity polynomial
                  K = 1 : odd polynomial  ( X(1) must be >  0 )
                  K = 2 : even polynomial ( X(1) must be >= 0 )
  EPSH    : tolerance for leveling. A useful value for 24-bit
            mantissa is EPSH = 2.0E-7
  MAXIT   : upper limit for number of exchange steps
  REF0(M2): initial alternating set ( N-vector ). This is an
            OPTIONAL argument. The length M2 is given by:
                  M2 = M + 2                      , if K = 0
                  M2 = integer part of (M+3)/2    , if K = 1
                  M2 = 2 + M/2 (M must be even)   , if K = 2

  OUTPUT VARIABLES:

  A       : polynomial coefficients of the best approximation
            in order of increasing powers:
                  p*(x) = A(1) + A(2)*x + A(3)*x^2 + ...
  REF     : selected alternating set of points
  HMAX    : maximum deviation ( uniform norm of p* - f )
  H       : pointwise approximation errors
	R		: total number of iterations
  EQUAL   : success of failure of algorithm
                  EQUAL=1 :  succesful
                  EQUAL=0 :  convergence not acheived
                  EQUAL=-1:  input error
                  EQUAL=-2:  algorithm failure

  Relies on function EXCH, provided below.

  Example: 
  M = 5; N = 10000; K = 0; EPSH = 10^-12; MAXIT = 10;
  X = linspace(-1,1,N);   % uniformly spaced nodes on [-1,1]
  k=1; Y = abs(X).^k;     % the function Y to approximate
  [A,REF,HMAX,H,R,EQUAL] = polyfitinf(M,N,K,X,Y,EPSH,MAXIT);
  p = polyval(A,X); plot(X,Y,X,p) % p is the best approximation

  Note: using an even value of M, e.g., M=2, in the example above, makes
  the algorithm to fail with EQUAL=-2, because of collocation, which
  appears because both the appriximating function and the polynomial are
  even functions. The way aroung it is to approximate only the right half
  of the function, setting K = 2 : even polynomial. For example: 

N = 10000; K = 2; EPSH = 10^-12; MAXIT = 10;  X = linspace(0,1,N);
for i = 1:2
    k = 2*i-1; Y = abs(X).^k;
    for j = 1:4
        M = 2^j;
        [~,~,HMAX] = polyfitinf(M,N,K,X,Y,EPSH,MAXIT);
        approxerror(i,j) = HMAX;
    end
end
disp('Table 3.1 from Approximation theory and methods, M.J.D.POWELL, p. 27');
disp(' ');
disp('            n          K=1          K=3'); 
disp(' '); format short g;
disp([(2.^(1:4))' approxerror']);

  ALGORITHM:

  Computation of the polynomial that best approximates the data (X,Y)
  in the discrete uniform norm, i.e. the polynomial with the  minimum
  value of max{ | p(x_i) - y_i | , x_i in X } . That polynomial, also
  known as minimax polynomial, is obtained by the exchange algorithm,
  a finite iterative process requiring, at most,
     n
   (   ) iterations ( usually p = M + 2. See also function EXCH ).
     p
  since this number can be very large , the routine  may not converge
  within MAXIT iterations . The  other possibility of  failure occurs
  when there is insufficient floating point precision  for  the input
  data chosen.

  CREDITS: This routine was developed and modified as 
  computer assignments in Approximation Theory courses by 
  Prof. Andrew Knyazev, University of Colorado Denver, USA.

  Team Fall 98 (Revision 1.0):
          Chanchai Aniwathananon
          Crhistopher Mehl
          David A. Duran
          Saulo P. Oliveira

  Team Spring 11 (Revision 1.1): Manuchehr Aminian

  The algorithm and the comments are based on a FORTRAN code written
  by Joseph C. Simpson. The code is available on Netlib repository:
  http://www.netlib.org/toms/501
  See also: Communications of the ACM, V14, pp.355-356(1971)

  NOTES:

  1) A may contain the collocation polynomial
  2) If MAXIT is exceeded, REF contains a new reference set
  3) M, EPSH and REF can be altered during the execution
  4) To keep consistency to the original code , EPSH can be
  negative. However, the use of REF0 is *NOT* determined by
  EPSH&lt; 0, but only by its inclusion as an input parameter.

  Some parts of the code can still take advantage of vectorization.  

  Revision 1.0 from 1998 is a direct human translation of 
  the FORTRAN code http://www.netlib.org/toms/501
  Revision 1.1 is a clean-up and technical update.  
  Tested on MATLAB Version 7.11.0.584 (R2010b) and 
  GNU Octave Version 3.2.4

</pre>

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