text: namespace index {
  
  size_t d2(size_t i_x1, size_t i_x2, size_t Nx2){
  
      size_t i_grid =   i_x1*Nx2
                   + i_x2;
  
      return i_grid;
  
  }
  
  size_t d3(size_t i_x1, size_t i_x2, size_t i_x3, size_t Nx2, size_t Nx3){
  
      size_t i_grid =   i_x1*Nx2*Nx3
                   + i_x2*Nx3
                   + i_x3;
  
      return i_grid;
  
  }
  
  } // index