Cells in a culture grow every day

We want to know the number of cells every day: `ncell[t]`

Here `t`

is the time in days.

We start with an initial number of cells, that we call `initial`

Each day, the number of cells increases by **a factor** `rate`

\[\texttt{ncell[t]} = \texttt{initial} \cdot \texttt{rate}^\texttt{t}\]