  One of the most intriguing findings in the structure of neural network
landscape is the phenomenon of mode connectivity: For two typical global
minima, there exists a path connecting them without barrier. This concept of
mode connectivity has played a crucial role in understanding important
phenomena in deep learning.
  In this paper, we conduct a fine-grained analysis of this connectivity
phenomenon. First, we demonstrate that in the overparameterized case, the
connecting path can be as simple as a two-piece linear path, and the path
length can be nearly equal to the Euclidean distance. This finding suggests
that the landscape should be nearly convex in a certain sense. Second, we
uncover a surprising star-shaped connectivity: For a finite number of typical
minima, there exists a center on minima manifold that connects all of them
simultaneously via linear paths. These results are provably valid for linear
networks and two-layer ReLU networks under a teacher-student setup, and are
empirically supported by models trained on MNIST and CIFAR-10.
