Mathematical equivalent of reduce()?
Map
applies a unary function to each element in the sequence and returns a new sequence containing the results, in the same order

$$\mathrm{map}\colon (E\to F) \times \mathrm{Seq}\langle E\rangle \rightarrow \mathrm{Seq}\langle F\rangle\,.$$

Filter tests each element with a unary predicate. Elements that satisfy the predicate are kept; those that don’t are removed.

$$\mathrm{filter}\colon (E \rightarrow \mathrm{boolean}) \times \mathrm{Seq}\langle E\rangle \rightarrow \mathrm{Seq}\langle E\rangle\,.$$

Reduce combines the elements of the sequence together, using a binary function. In addition to the function and the list, it also takes an initial value that initializes the reduction, and that ends up being the return value if the list is empty.

$$\mathrm{reduce}\colon (F \times E \rightarrow F) \times \mathrm{Seq}\langle E\rangle \times F \rightarrow F\,.$$

Example:

\begin{align*}
\mathrm{result}_0 &amp;= \mathrm{init} \\
\mathrm{result}_1 &amp;= f(\mathrm{result}_0, \mathrm{list}[0]) \\
\mathrm{result}_2 &amp;= f(\mathrm{result}_1, \mathrm{list}[1]) \\
&amp;\ \ \vdots\\
\mathrm{result}_n &amp;= f(\mathrm{result}_{n-1}, \mathrm{list}[n-1])
\end{align*}

Hence you can see it as a function $\mathbb{R}^n \rightarrow \mathbb{R}$

Source: http://web.mit.edu/6.005/www/fa15/classes/25-map-filter-reduce/