function f=cosef(ww,x,i)
 
  $Id: cosef.m,v 1.14 2003-06-22 01:02:05-05 brinkman Exp $
  (c) 2002, Peter Brinkmann (brinkman@math.uiuc.edu)
 
  cosef: an implementation of the interface for eigenfunctions suitable
 	for use with coeff.m
 
  Note that these eigenfunctions do _not_ need to be normalized because
  the module coeff.m takes care of this.
 
  Usage: ef=cosef(2,linspace(0,pi,30),3);
 
  Parameters:
    ww: square root of eigenvalue or column vector of roots of eigenvalues
    x: scalar, or row vector or matrix of x-values
    i: index or column vector of indices indicating the location of the
 		entry or entries of ww in the list of all eigenvalues
 
  In order to conform to the interface for eigenfunctions, a function must
  return a meaningful value whenever the product ww*x is defined. This is
  the case
 	- if ww is a scalar and x is anything, or
 	- if x is a scalar and ww is anything, or
 	- if both ww and x are vectors.
  Note that in the third case, the product ww*x is the outer product of two
  vectors. When writing rather basic eigenfunctions that only depend on the
  product ww*x, this will not concern you. However, when writing a
  sophisticated function whose result doesn't just depend on ww*n but on
  the index i as well, then you need to make sure that the outer product
  case is handled correctly. See the file periodicef.m for an example of
  how to deal with outer products.
 
  Return values:
    f: cos(ww*x)