  McDuff and Schlenk determined when a four-dimensional ellipsoid can be
symplectically embedded into a ball, and found that part of the answer is given
by an infinite "Fibonacci staircase." Similarly, Frenkel and M\"uller
determined when a four-dimensional ellipsoid can be symplectically embedded
into the ellipsoid E(1,2) and found that part of the answer is given by a "Pell
staircase." ECH capacities give an obstruction to symplectically embedding one
four-dimensional ellipsoid into another, and McDuff showed that this
obstruction is sharp. We use this result to give new proofs of the staircases
of McDuff-Schlenk and Frenkel-M\"uller, and we prove that another infinite
staircase arises for embeddings into the ellipsoid E(1,3/2). Our proofs relate
these staircases to a combinatorial phenomenon of independent interest called
"period collapse" of the Ehrhart quasipolynomial.
  In the appendix, we use McDuff's theorem to show that for a >= 6, the only
obstruction to embedding an ellipsoid E(1,a) into a scaling of E(1,3/2) is the
volume, and we also give new proofs of similar results for embeddings into
scalings of E(1,1) and E(1,2).
