add reference for g_sync_finite_group method

Author: j-c-c

src/aspire/abinitio/commonline_utils.py

@@ -412,7 +412,7 @@ def g_sync_cyclic(rots, rots_gt, symmetry):
 
 def g_sync_finite_group(rots, rots_gt, symmetry):
     """
-    Synchronize ground-truth rotations over a finite symmetry group.
+    Synchronize ground truth rotations over a finite symmetry group.
 
     This is a finite-group generalization of cyclic synchronization. The
     pairwise matching step estimates relative symmetry elements between image
@@ -423,6 +423,12 @@ def g_sync_finite_group(rots, rots_gt, symmetry):
     as a scalar complex phase. For non-commutative groups such as D_n, we instead
     represent each group element by its left-regular permutation matrix.
 
+    This follows the standard spectral group-synchronization template: build a
+    block matrix of pairwise group measurements, compute the leading group-sized
+    eigenspace, and round the recovered blocks back to group elements. See Ling,
+    "Near-Optimal Performance Bounds for Orthogonal and Permutation Group
+    Synchronization via Spectral Methods", Appl. Comput. Harmon. Anal. 2022.
+
     :param rots: Estimated rotation matrices
     :param rots_gt: Ground truth rotation matrices
     :param symmetry: The symmetry of the underlying molecule.