function [ alpha, beta, cstar, delta, epsilon, gamma, k, lambda, mu, t0, ...
  tstop, xmin, xmax ] = tumor_parameters ( alpha_user, beta_user, ...
  cstar_user, delta_user, epsilon_user, gamma_user, k_user, lambda_user, ...
  mu_user, t0_user, tstop_user, xmin_user, xmax_user )

%*****************************************************************************80
%
%% tumor_parameters() returns the parameters of the tumor PDE.
%
%  Discussion:
%
%    If input values are specified, this resets the default parameters.
%    Otherwise, the output will be the current defaults.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    04 May 2021
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real ALPHA_USER:
%
%    real BETA_USER:
%
%    real CSTAR_USER:
%
%    real DELTA_USER:
%
%    real EPSILON_USER:
%
%    real GAMMA_USER:
%
%    real K_USER:
%
%    real LAMBDA_USER:
%
%    real MU_USER:
%
%    real T0_USER: the initial time.
%
%    real TSTOP_USER: the final time.
%
%    real XMIN_USER: the minimum X value.
%
%    real XMAX_USER: the maximum X value.
%
%  Output:
%
%    real ALPHA:
%
%    real BETA:
%
%    real CSTAR:
%
%    real DELTA:
%
%    real EPSILON:
%
%    real GAMMA:
%
%    real K:
%
%    real LAMBDA:
%
%    real MU:
%
%    real T0: the initial time.
%
%    real TSTOP: the final time.
%
%    real XMIN: the minimum X value.
%
%    real XMAX: the maximum X value.
%
  persistent alpha_default;
  persistent beta_default;
  persistent cstar_default;
  persistent delta_default;
  persistent epsilon_default;
  persistent gamma_default;
  persistent k_default;
  persistent lambda_default;
  persistent mu_default;
  persistent t0_default;
  persistent tstop_default;
  persistent xmin_default;
  persistent xmax_default;
%
%  Initialize defaults.
%
  if ( isempty ( alpha_default ) )
    alpha_default = 10.0;
  end

  if ( isempty ( beta_default ) )
    beta_default = 4.0;
  end

  if ( isempty ( cstar_default ) )
    cstar_default = 0.2;
  end

  if ( isempty ( delta_default ) )
    delta_default = 1.0;
  end

  if ( isempty ( epsilon_default ) )
    epsilon_default = 0.001;
  end

  if ( isempty ( gamma_default ) )
    gamma_default = 1.0;
  end

  if ( isempty ( k_default ) )
    k_default = 0.75;
  end

  if ( isempty ( lambda_default ) )
    lambda_default = 1.0;
  end

  if ( isempty ( mu_default ) )
    mu_default = 100.0;
  end

  if ( isempty ( t0_default ) )
    t0_default = 0.0;
  end

  if ( isempty ( tstop_default ) )
    tstop_default = 0.7;
  end

  if ( isempty ( xmin_default ) )
    xmin_default = 0.0;
  end

  if ( isempty ( xmax_default ) )
    xmax_default = 1.0;
  end
%
%  Update defaults if input was supplied.
%
  if ( 1 <= nargin )
    alpha_default = alpha_user;
  end

  if ( 2 <= nargin )
    beta_default = beta_user;
  end

  if ( 3 <= nargin )
    cstar_default = cstar_user;
  end

  if ( 4 <= nargin )
    delta_default = delta_user;
  end

  if ( 5 <= nargin )
    epsilon_default = epsilon_user;
  end

  if ( 6 <= nargin )
    gamma_default = gamma_user;
  end

  if ( 7 <= nargin )
    k_default = k_user;
  end

  if ( 8 <= nargin )
    lambda_default = lambda_user;
  end

  if ( 9 <= nargin )
    mu_default = mu_user;
  end

  if ( 10 <= nargin )
    t0_default = t0_user;
  end

  if ( 11 <= nargin )
    tstop_default = tstop_user;
  end

  if ( 12 <= nargin )
    xmin_default = xmin_user;
  end

  if ( 13 <= nargin )
    xmax_default = xmax_user;
  end
%
%  Return values.
%
  alpha = alpha_default;
  beta = beta_default;
  cstar = cstar_default;
  delta = delta_default;
  epsilon = epsilon_default;
  gamma = gamma_default;
  k = k_default;
  lambda = lambda_default;
  mu = mu_default;
  t0 = t0_default;
  tstop = tstop_default;
  xmin = xmin_default;
  xmax = xmax_default;

  return
end

