50  const int idx2 = idx -1;
 
   51  index[3] = idx2/(lx * lx * lx) ;
 
   52  index[2] = (idx2 - (lx*lx*lx)*index[3])/(lx * lx);
 
   53  index[1] = (idx2 - (lx*lx*lx)*index[3] - (lx*lx) * index[2]) / lx;
 
   54  index[0] = (idx2 - (lx*lx*lx)*index[3] - (lx*lx) * index[2]) - lx*index[1];