Walrasian economy9
endowment of commodityiis denoted by¯c
ai,¯cai≥0,i=1,...,T. Naturally,
m
≥
a=1
¯cai=¯ci>0.
Note that this assumption reflects an exchange economy in which private property
rights exist. “Private property rights” means that for each unit of each good, the
exclusive right to determine use has been assigned to a particular individual.
Assumption 2.2All exchanges occur at a single point in time.
Assumption 2.3Each individual confronts the same known set of prices at which
exchange can occur.
4
Arelative(purchase iindicates the units
of commodityjrequired to purchase one unit of commodityi.Arelative(sale)
price of commodityiindicates units of commodityjreceived when one unit of
commodityiis sold.
Assumption 2.4Purchase and sale prices are identical for each commodity. This
means that there are no “price spreads” which would suggest either a gain to an
individual buying and selling the same commodity or the presence of costs to
making an exchange.
5
Thus, for theTcommodities there areT
2
exchange rates,
or relative prices, taking two commodities at a time.
The numeraire
While there areT
2
exchange rates, the complete set of exchange rates can be
deduced directly or indirectly by the set ofT−1 relative prices:
(π
1j,...,π j−1,j,πj+1,j,...,π Tj),
where theπ
ijdenotes the price of commodityiin terms of commodityj.
6
In the
listing of relative prices,(π
1j,...,π j−1,j,πj+1,j,...,π Tj), commodityjis referred
to as the “numeraire.”
To see how the set of relative prices reduces toT−1, we rely on the fact
thatπ
hh=πhj/πhjfor allh,j, andk. Let us see what this means for a simple
example of three commodities,h,j, andk. There are thenT
2
or nine different
relative prices, which are:π
hh,πjj,πkk,πhj,πjh,πhk,πkh,πjk, andπ kj. But, we can
use the relationshipπ
hh=πhj/πhjto reduce this toT−1=2 relative prices with
informational content. In particular, we know that:
1π
hh,πhk/πhj, and similarly forπ jj, andπ kk, whenh=j=k. In other words,
the exchange rate of a commodity with itself is unity. This reduces from nine
to six the number of relative prices for which information is required.
2π
jh=1/π hjwhenk=j(such thatπ kj=πkk=1). Similarlyπ hk=1/π kh
andπ jk=1/π kj. For example, if thejth commodity is pears and thehth