# A difficulty in understanding a case in the intersection theory mod 2(p.80 Guillemin and Pollack)

The following is written just before the boundary theorem in Guillemin & Pollack :

But I see that if Z is not transversal to X this is not true,why the book did not consider this case? why the book build the boundary theorem on the transversality case only? could anyone explain this for me please?

Boundary Theorem: suppose that $$X$$ is the boundary of some manifold $$W$$ and $$g: X \to Y$$ is a smooth map. If $$g$$ may be extended to all of $$W$$ then $$I_2 (g,Z)=0$$ for any closed submanifold $$Z$$ in $$Y$$ of complementary dimension.

• Aren't these things only defined in the transverse case, because otherwise there's too many pathologies? Then there's some theorem that says you can homotope a non-transverse case into a transverse one or something. – Randall Dec 9 '18 at 2:57
• yes you are right Randall @Randall – hopefully Dec 9 '18 at 2:59