Let $M$ be an orientable closed 3-manifold and suppose $A$ and $B$ are embedded incompressible closed orientable surfaces in $M$ with $[A] = [B]$ in $H_2(M,\mathbb{Z})$.
In general, there are a variety of obstructions to the existence of a cobordism between two representatives of a homology class. However, I have a strong memory of reading a paper that states that in this particular setting one can say something quite strong. The result I remember reading is that: there exists a sequence $C_0,C_1,\ldots, C_k$ of embedded surfaces in $M$ and submanifolds $N_1,\ldots N_k$ such that:
- $A = C_0$
- $B = C_k$
- $N_i$ is a cobordism from $C_{i-1}$ to $C_i$
Unfortunately, I have been unable to locate this paper or recall the author. I would appreciate it very much if someone recalls the reference (or can tell me my memory is wrong and this is false).