Differential equation help please

bmulla11

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May 23, 2020
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Hi, I've been given a tricky question and need help solving. Thanks in advance!

This is a simplified variant of the neoclassical growth model. Here we assume(contrary to the Solow growth model) that labor L does not change over time and that L = 1. We assume Y = 5/2 * K(1/5), where Y is output and K is capital. We assume for the change in capital stock dK/dt = 1/2 * Y. Consider the resulting differential equation for capital:
dK/dt =5/4 * K(1/5).

We are interested in the time path of K from time t = 0 onwards. Assume that K(0) = 0. Then there is an obvious solution, namely K(t) = 0 for all t ≥ 0. Hence, for this solution, K = 0 is a steady state! But there is also another solution such that K(0) = 0, but K(t) > 0, for all t > 0. For this alternative solution K = 0 is not a steady state!

a) Please find this alternative solution!
 
Assuming K is NOT 0, we can divide by it and the differential equation, \(\displaystyle \frac{dK}{dt}= \frac{5}{4}K^{1/5}\) can be separated as \(\displaystyle K^{-1/5}dK= \frac{5}{4}dt\). Integrating both sides, \(\displaystyle \frac{5}{4}K^{4/5}= \frac{5}{4}t+ C\). We can write that as \(\displaystyle K^{4/5}= t+ C'\) where \(\displaystyle C'= \frac{4}{5}C\). With K(0)= 0 that becomes C'= 0 so \(\displaystyle K^{4/5}= t\) or \(\displaystyle K= t^{5/4}\(\displaystyle .\)\)
 
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