Homotopy classification of projections in the corona algebra of a non-simple $C\text{sp}\ast$-algebra

We study projections in the corona algebra of $C(X)\otimes K$ where $X=[0,1],[0,\mathrm{\infty }),(-\mathrm{\infty },\mathrm{\infty })$, and $[0,1]\mathrm{/}\{0,1\}$. Using BDF’s essential codimension, we determine conditions for a projection in the corona algebra to be liftable to a projection in the multiplier algebra. Then we also characterize the conditions for two projections to be equal in $K_0$-group, Murray-von Neumann equivalent, unitarily equivalent, and homotopic from the weakest to the strongest. In light of these characterizations, we construct examples showing that any two equivalence notions do not coincide, which serve as examples of non-stable K-theory of $C\text{sp}\ast$-algebras.