Marginal Product of Capital in the Solow Model In the classic form of the Solow Model:

$$ Y=K^\alpha (AL)^{1-\alpha } $$

Describe circumstances in which the marginal product of capital could rise over time, at least for a temporary period.

I've calculated:

$$ MPK = \frac{dY}{dK} = \alpha K^{\alpha -1}(AL)^{1-\alpha } $$

The thing is, I thought one basic assumptions of the Solow Model was diminishing marginal returns, so how could there ever be a case when MPK is increasing?
According to your calculations MPK is not increasing in $K$. The Solow model assumes $0&lt; \alpha &lt; 1$, thus $\alpha - 1 &lt; 0$ and $K^{\alpha - 1}$ is decreasing in $K$.