The parent_directory function returns the name of the directory that's located just above the current working directory. Remember that '..' is a relative path alias that means "go up to the parent directory". Fill in the gaps to complete this function.
The key here is that this does not literally append '..' inside the current directory path. Instead, the '..' still acts as “move up” in the directory hierarchy.
The colon acts as a separator from the field name, if you had specified one.
if not following the variable order, you can specify the variable here
Geometric proof
A way to derive all the five Platonic Polyhedra
Each vertex of the solid must be a vertex for at least three faces.
How this was derived? Why it has to be three? because two faces can not form a volume.
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the "Modern Form" always failed when I tried to search with the titles of the papers