Demand for capital by firms must satisfy
Et[(1 + Rt+1)RPSt+1] =
Et h(1^^ α) ∣+. + + (1^^ δ)Qt+ɪi
(15)
Qt
In (15) the right-hand-side is the gross return to holding a unit of capital in from t to t + 1.
The left-hand-side is the gross return from holding bonds, the opportunity cost of capital
and includes an exogenous risk-premium shock RPSt, which, for now, we leave unmodelled.
We complete the set-up with investment costs by defining the functional form
S(X) =φX(Xt-(1+g))2
(16)
The RBC model we have set out defines a equilibrium in output, Yt , consumption Ct ,
investment It , capital stock Kt and factor prices, Wt for labour and Rt for capital, and the
price of capital Qt , given exogenous processes for technology At , government spending Gt
and the risk premium shock RP St.
2.2 From RBC to NK
The NK framework combines the DSGE characteristics of RBC models with frictions such as
monopolistic competition - in which firms produce differentiated goods and are price-setters,
instead of a Walrasian determination of prices, and nominal rigidities, in which firms face
constraints on the frequency with which they are able to adjust their prices. Therefore,
we now introduce a monopolistically competitive retail sector that uses a homogeneous
wholesale good to produce a basket of differentiated goods for consumption
Ct =
μjι Ct(m)(ζ-ɪ)/ζdm)
(17)
where ζ is the elasticity of substitution. This implies a set of demand equations for each
intermediate good m with price Pt(m) of the form
Ct ( m )= μ PtPm ) ζ Ct
(18)
where Pt = [Rql Pt(m)ɪ-ζdm 1 z is the aggregate price index. A competitive capital-
producing sector is modelled as for the RBC model
Conversion of good m from a homogeneous output requires a cost cYtW (m) where whole-
sale production uses the production technology (8). Thus
Yt(m) = (1 - c)YtW (m)
(19)
(20)
YtW = ( Atht ) αK1-α