Lanczos' tau method for linear differential problems.

   a = tausolver(x, y, problem, conditions, varargin)

   a = tausolver(varargin)returns the coefficients, on basis P, of the 
   (n-1)th degree polinomial approximation yn = Pa, of the
   linear differential problem dy/dx=F(t,y). 

Input (required)
   x              = independent tau variable (itau object).
   y              = dependent tau variable (dtau oject).
   problem        = ordinary differential problem (char).
   conditions     = problem conditions (cell of char).

Inputs (optional)
   pieces         = number of steps for piecewise approach.
   exact_solution = exact solution (char). (integer)
   step           = step on the x vector to show the results.
   precond        = preconditioner ('no', 'ilu', 'diag').
                    for 'ndiad' define: 'numbd' (number of diagonals);
                    for 'ilu' define: 'milu', 'typeilu', 'droptol', 
                                      'thresh' and 'udiag'.
   solver         = linear system solver.
   apsol          = boolean varargin to show the graphical solution.
   resid          = boolean varargin to show the graphical error.
   coeff          = boolean varargin to show the coefficients a.
   spy            = boolean varargin to spy the T matrix.
   saves          = name varargin to save the results at .mat.

   a              = approximate solution coefficients at basis P.

See also
  tauode, taupw, tausys, tausyspw and schursolver.