Assume that there are $26$ points $s_1, s_2, \ldots, s_5, d, n_1, n_2, \ldots, n_{20}$, where $s_i$'s are source points, $d_1$ is a destination point, $n_i$'s are normal points.
Let us denote the position of source point $s_i$'s by $p_{s_i}$.
Let us denote the position of destination point $d$'s by $p_{d}$.
Let us denote the position of normal points $n_i$'s by $p_{n_i}$.
All positions are in $\mathbb{R}^2$.
For example, $p_{s_1}=(1,0),~\ldots,~p_{n_{20}}=(50, 30)$.
I want to express a function $f$ whose inputs are a set of points of all points and whose output is a set of vectors of some points (a path from each source to its corresponding destination).
For example, $$f(p_{s_1},\ldots,p_{s_5},p_{d},p_{n_1},\ldots,p_{n_{20}})=\{f_{s_1}, f_{s_2}, f_{s_3}, f_{s_4}, f_{s_5}\},$$ where $f_{s_i}=(p_{s_i},\ldots,p_{d})$ for all $i\in\{1,2,3,4,5\}$. Here, '$\dots$' are normal nodes arbitrarily determined according to a specific rule.
In this case, the function $f$ is defined by $$ f:\mathbb{R}^{2\times26} \to ???. $$
I want to define this function, but it is too hard to express in formal paper. If someone know how to express this function, please let me know, thank you.
Denote by $N$ your set of 20 normal nodes, and by $N^k$, the $k$-fold cartesian product of $N$ with itself. Now define $\mathcal N$ as $$\mathcal N = N \cup N^2 \cup N^3 \cup \cdots$$ Now your function $f$ must be on the same domain you have given, but the codomain should be $\mathcal N$.
Suppose, for example, $f(s_2)$ is an element of $N^7$, this means this function selects a path from $f_{s_2}$ to $p_d$ having 7 intermediate normal nodes. I hope my understanding of your question is correct.