At times a book has a dependency diagram, the order that chapters can be tackled, from chapters that require no other chapters (i.e. the simplest chapters), to chapters that might rely on knowledge of 3,4 other chapters (most complex chapters).
Is there a dependency graph for mathematical subjects? i.e. what subjects $s_1,\cdots s_n$ are needed to be known before subject $t$ can be taken?
Such a dependency would be useful for self study