function r = real_roots ( a, b, c )

  d = b^2 – 4.0 * a * c;

  if ( d < 0.0 )
    r = [];
  elseif ( d == 0.0 )
    r = - b / ( 2.0 * a );
  else
    r = [ ( - b + sqrt ( d ) ) / ( 2.0 * a ), ...
          ( - b – sqrt ( d ) ) / ( 2.0 * a ) ];
  end

  return
end

