# What are the functionality of δ symbol and $δr^T$?

I got two questions here:

1. Does anybody know what is the functionality of the small delta letter δ here? Is it simply the same as the rate of change just like the big delta letter Δ?

2. And for the $δr^T$, does T have any special definition in mathematics or should we simply treat it as usual power variable?

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The $\delta r$ is the differential of $r$. So $r_0 + \delta r$ represents a very small change away from $r_0$. See this page for more on infinitesimal differentials. When differentials are usually small, $\delta$ is used instead of $\Delta$.
The $T$ is the transpose of a matrix. In this case, $r$ is a vector and in order to get the matrix multiplication to line up correctly, it's necessary to take the transpose of $r$.