A MAGNETIC FRAMELET-BASED CONVOLUTIONAL NEURAL NETWORK FOR
DIRECTED GRAPHS
Lequan Lin and Junbin Gao
Discipline of Business Analytics, The University of Sydney Business School
The University of Sydney, Camperdown, NSW 2006, Australia
llin0615@uni.sydney.edu.au, junbin.gao@sydney.edu.au
ABSTRACT
Spectral Graph Convolutional Networks (spectral GCNNs),
a powerful tool for analyzing and processing graph data,
typically apply frequency filtering via Fourier transform to
obtain representations with selective information. Although
research shows that spectral GCNNs can be enhanced by
framelet-based filtering, the massive majority of such re-
search only considers undirected graphs. In this paper, we
introduce Framelet-MagNet, a magnetic framelet-based spec-
tral GCNN for directed graphs (digraphs). The model applies
the framelet transform to digraph signals to form a more
sophisticated representation for filtering. Digraph framelets
are constructed with the complex-valued magnetic Laplacian,
simultaneously leading to signal processing in both real and
complex domains. We empirically validate the predictive
power of Framelet-MagNet over a range of state-of-the-art
models in node classification, link prediction, and denoising.
Index Terms—Directed Graph, Graph Convolutional
Neural Network, Magnetic Laplacian, Graph Framelets,
Graph Framelet Transform
1. INTRODUCTION
Recent years have witnessed the surging popularity of re-
search on graph convolutional neural networks (GCNNs)
[1]. Through the integration of graph signals and topolog-
ical structures in graph convolution, GCNNs usually pro-
duce more valuable insights than models that analyze data
in isolation. Especially, spectral GCNNs define their graph
convolution in the frequency domain, enabling the filtering of
different frequency components in graph signals. However,
the majority of studies on signal processing in spectral GC-
NNs only focus on undirected graphs [2, 3, 4]. In this paper,
we aim to extend framelet-based signal processing to spectral
GCNNs for directed graphs (digraphs).
Many directional relationships are naturally modelled
as digraphs, such as citation relationships [5], website hy-
perlinks [6], and road directions [7]. Using graph edges
to represent directional information backbones the explo-
ration of more aspects of the underlying data, which usually
provides more useful findings. Nonetheless, while spec-
tral GCNNs assume the eigendecomposition of symmetric
graph Laplacian to provide real-valued eigenvalues and or-
thonormal eigenvectors, the digraph Laplacian is asymmetric.
Converting digraphs to undirected graphs facilitates the ex-
tension of spectral methods to digraphs, but destroys the
digraph structure. Therefore, many recent studies engage
in designing symmetric digraph Laplacian that can preserve
the directional information [8, 9, 10, 11]. Magnetic Lapla-
cian [11, 12] is one of the most successful instances. It is
a complex-valued Hermitian matrix, whose real part shows
edge existence, and imaginary part indicates edge directions.
Magnetic Laplacian-based digraph networks exploit magnetic
Laplacian in classic spectral GCNN architectures and have
demonstrated their power in various graph tasks [11].
Classic spectral GCNNs adopt Fourier transform in their
convolutional layer. Converting graphs signals to the Fourier
frequency domain allows the processing of signal frequen-
cies, but only from a global perspective. More specifically,
although we can detect signal frequencies, we cannot iden-
tify their position in the graph. To investigate both global
and local information, we can ensemble spectral GCNNs with
framelet transform instead. The framelet frequency domain is
composed of graph framelets, which are constructed through
dilation and translation of a set of localized scaling functions.
Nevertheless, most existing wavelet/framelet-based networks
are solely applicable to undirected graphs [2, 3, 4]. Although
SVD-GCN [13] accomplishes digraph framelet transform via
singular value decomposition (SVD) of the asymmetric di-
graph Laplacian, its theoretical rationale is very vague, for
example, how the Laplacian frequency can be linked to the
signal frequency in the SVD domain.
In this paper, we propose Framelet-MagNet, a magnetic
Laplacian-assisted framelet-based spectral GCNN for di-
graphs. Multiresolution Analysis enables us to construct
digraph framelets with the magnetic Laplacian and a filter
bank [14, 15]. In addition, we also construct quasi-framelet
directly in the frequency domain to impose double regulation
on digraph signals [4]. We exploit Chebyshev polynomial
approximation for fast framelet transform and reconstruction.
arXiv:2210.10993v2 [cs.LG] 2 May 2023