According to theorems of C.
Gordon, J. Luecke, and W. Parry, if a knot exterior X has two distinct planar
boundary slopes r1,r2, then at least one of the manifolds X(r1),X(r2) has a
connected summand M with nontrivial torsion in first homology. The 3-manifolds
M obtained in this way, which we call t-manifolds, have special Heegaard
splittings, or t-manifold structures. In this paper we study the topology of
t-manifolds from the point of view of the homology presentation matrices
induced by their t-manifold structures, classify all genus two t-manifold
structures, and show that, under some conditions, one of the Dehn fillings of X is
a connected sum of t-manifolds and (at most) one prime non t-manifold
summand.
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