Reference matrix:
 1.50	 2.00	 0.50	 0.00	 0.00	
 2.50	 5.00	 3.50	 0.50	 0.00	
 0.50	 4.00	10.00	 5.00	 0.50	
 0.00	 0.50	 5.50	17.00	 6.50	
 0.00	 0.00	 0.50	 7.00	25.50	
sprspm matrix (should show only tridiagonals):
 1.50	 2.00	 0.00	 0.00	 0.00	
 2.50	 5.00	 3.50	 0.00	 0.00	
 0.00	 4.00	10.00	 5.00	 0.00	
 0.00	 0.00	 5.50	17.00	 6.50	
 0.00	 0.00	 0.00	 7.00	25.50