Hi, everyone:

I am trying to work with the intersection form in 4-manifolds. Specifically,

I am working with CP^2 (complex projective 2-space.), whose form is given by <1>.

  Now, I know how to compute an actual numerical value when we work with the form

in cohomology: we cup-product two cochains a,b , and then evaluate a/b on the

fundamental class.

   But when we work in homology (using Poincare Duality) , I am not too clear

 on how we actually get a number by starting with a matrix (we always have

 representative surfaces for 2-homology in a 4-manifold.). What do we 

 evaluate this matrix in.?

  Thanks.