  We propose a general theoretical framework for both constructing and
diagnosing inversion-protected higher-order topological superconductors using
Kitaev building blocks, a higher-dimensional generalization of Kitaev's
one-dimensional Majorana model. For a given crystalline symmetry, the Kitaev
building blocks serve as a complete basis to construct all possible Kitaev
superconductors that satisfy the symmetry requirements. We derive a simple yet
powerful Majorana counting rule that can unambiguously diagnose the existence
of higher-order topology for all Kitaev superconductors. We expect this
real-space diagnosis to work for general two-dimensional higher-order
topological superconductors within this symmetry class. As proof of concept, we
have identified two inequivalent stacking strategies using the Kitaev building
blocks, based on which we have constructed minimal tight-binding models with
symmetry-protecetd Majorana corner modes. Moreover, we have successfully
applied our diagnosis to comprehend the Majorana corner physcis in a
superconductor model with a fragile Wannier obstruction, confirming the
validity of our theory beyond the Kitaev limit. Our work paves the way for
interpreting higher-order topological superconductivity from the real-space
perspective.
