buildCycleLaplacian

Builds the graph Laplacian of an n-node cycle graph (node i connected to i - 1 and i + 1, indices modulo n) -- the discrete analogue of the periodic 1D Laplacian, and the natural graph to put on n points sampled around a closed curve. Symmetric positive semi-definite, with the constant vector as its null (zero-eigenvalue) eigenvector -- see extractCycleEigenModes.

Uses unit edge weights throughout (equivalently, unit spacing between nodes): since only eigenvalue ratios are exposed via GridEigenMode.frequencyRatio-style results, the physical spacing of the original samples doesn't need to be carried through, it only rescales every eigenvalue by the same constant factor.

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