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With the exception of the
hyperpyramidal numbers (which include the
pyramidal numbers and the
centered pyramidal numbers, i.e. the
(centered polygons) pyramidal numbers, as
3-dimensional hyperpyramidal numbers), all the
figurate numbers considered are
regular polytope numbers corresponding to regular convex polytopes in a
-dimensional Euclidean space
Among the
hyperpyramidal numbers, the
-dimensional
square hyperpyramidal numbers, although not corresponding to
regular polytopes, are of particular interest since they are building blocks for the construction of the
hyperoctahedral numbers (
orthoplicial polytopic numbers), which are regular polytopes. For example, the
th octahedral number is the
th square dipyramidal number, i.e. it is the adjunction of the
th square pyramidal number to the
th square pyramidal number (corresponding to joined
square pyramids at their square bases), while for
hyperoctahedral numbers of dimension
we must do
successive adjunction operations.
Otherwise, considering nonconvex regular (e.g. stellated) polytopic numbers or considering nonregular (e.g. Archimedean solids) polytopic numbers would open the door to a humongous number of possibilities...