The graphon fourier transform of a graphon signal(W,X) is a functional X^=WFT(X) defined as
X^j=X^(λj)=∫01X(u)φj(u)du
where λj are the eigenvalues of W and {φi} are the eigenfunctions.
Why is the graphon fourier transform always defined?
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The eigenvalues λj are countable
Why are the eigenvalues of a graphon signal countable?
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This is a direct application of the spectral theorem for self-adjoint compact operators on Hilbert spaces