Synthetic topos theory
[0054] Exercise

Let UU be a universe.

  • -(Q(s1),Q(s0)):Q(Δ[2])Q(Δ[1])×Q(Δ[1])(\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{1}),\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{0})):\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 2\rbrack )\rightarrow \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 1\rbrack )\times \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 1\rbrack ) determines a partial order in Topos(U)\mathord {\textnormal {\textsf {Topos}}}(U). We refer to this partially ordered object in Topos(U)\mathord {\textnormal {\textsf {Topos}}}(U) as I\mathbb {I}.
  • -Q(d1):Q(Δ[0])Q(Δ[1])\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {d}}}^{1}):\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 0\rbrack )\rightarrow \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 1\rbrack ) determines the least element of I\mathbb {I}.
  • -Q(d0):Q(Δ[0])Q(Δ[1])\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {d}}}^{0}):\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 0\rbrack )\rightarrow \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 1\rbrack ) determines the greatest element of I\mathbb {I}.
  • -0Q(Δ[0])Q(d1)×Q(d0)Q(Δ[0])\mathord {\textnormal {\textsf {0}}}\simeq \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 0\rbrack )\mathbin {{}_{\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {d}}}^{1})}\mathord {\times _{\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {d}}}^{0})}}}\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 0\rbrack )
  • -Let n3n\ge 3 be an integer. Then the morphism
    (Q(sn1),(Q(s0))n2):Q(Δ[n])Q(Δ[n1])(Q(s0))n2×Q(s1)Q(Δ[2])(\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{n-1}),{(\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{0}))}^{n-2}):\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack n\rbrack )\rightarrow \mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack n-1\rbrack )\mathbin {{}_{{(\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{0}))}^{n-2}}\mathord {\times _{\mathord {\textnormal {\textsf {Q}}}(\mathord {\textnormal {\textsf {s}}}^{1})}}}\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack 2\rbrack )
    is an equivalence. (This means that Q(Δ[n])\mathord {\textnormal {\textsf {Q}}}(\Delta \lbrack n\rbrack ) is the object of increasing sequences in I\mathbb {I} of length nn).