We introduce a new topological invariant ν(X)∈ℕ∪{0} of compact manifolds-with-boundaries X which is much connected with boundary-unions. A boundary-union is a kind of decomposition of compact manifolds-with-boundaries. See the body of the paper for the precise definition. Let M and N be m-dimensional compact manifolds-with-boundaries. Let M∪∂N be a boundary-union of M and N. Then we have
We define
ν(X) as follows: First, define an invariant of
(n−1)-closed manifolds. Take the maximum of the invariant of all connected-components of the boundary of each handle-body of an ordered-handle-decomposition with a fixed base
A, where we impose the condition that the base
A is a (not necessarily connected) closed manifold. Take the minimum of the maximum for all ordered-handle-decompositions with the base
A. It is our another invariant
ν(X,A). Take the maximum of the minimum,
ν(X,), for all basis to satisfy the above condition. It is
ν(X). See the body of the paper for the precise definition.