1068 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 2X 
growth. It has often been suggested that such expenditure should 
be regarded as a part of productive investment. As a quantitative 
measure of the total stock of such investment at any given point of 
time one should possibly use the integral, i.e., the cumulated amount 
of educational expenditure incurred annually over the preceeding say 
thirty or fifty years. 
+3] Leontief - pag. 30