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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
principle. Often enough, an outlay which will to amortised 
over 18 months will still be charged against operating expen- 
ses. Let 1; denote the practical limit. 
Secondly, investments whose amortisation period is below 
t; and which are accounted as operating expenses contain 
both primary inputs and earlier inputs incorporated in the 
existing supplies and equipment. Thus, minor maintenance 
charges include at the same time wages, supplies produced 
some time earlier and the reward for the use of equipment. 
Thus the maintenance outlays which emerge into consumed 
income before 1; include not only the primary inputs supplied 
at moment ¢ but also primary inputs corresponding to goods 
produced earlier. The primary inputs of moment £ enter con- 
sumed income before 1, and at least as a first approximation, 
the second group of inputs can be considered as entering con- 
sumed income before (1, +6) where © is the average length 
nf the production period with the year as time unit (%). 
It may be assumed, at least as a first approximation that 
the shares of the two categories of input are respectively pro- 
portional to 
(334-23) A - 
Ro 
N= 
[f we consider the weighted average 
(334-24)  O'=AT,+ (1-1) (1, +0)=1, + (1-1) 
1) 0 is defined by relation (112-2). 
2) Relation (333-5). 
11] Allais - pag. 158