A fun byproduct of my Master's thesis: symmetric tensor functions are dense in the space of permutationinvariant multivariate functions
I prepared this post a long while ago but only posted it here in July. This is because I had written it in LaTeX and converting it to MD was not completely trivial. Since I had no incentive to post it, this small barrier was enough for me to procrastinate several months…
I’m not completely satisfied with the math rendering, so here is the LaTeX version.
During my Master’s thesis, I encountered several interesting technical points that were not directly related to the thesis topic, so that I left them hanging. Here I will talk about one of them, encountered while working on Volterra series. (I chose to jump directly into my main point without giving any context on Volterra series, as it is not necessary; for a clean introduction to these objects, see chapter 4 of my Master’s thesis report.)
 Preliminaries
 The result and why it looks surprising to me
 Brief proof of the result
 Is the result interesting/useful?
Preliminaries
Notations and shorthands Fix some integer \(n > 0\).

For a point \(\boldsymbol{t}= (t_1,...,t_n) \in \mathbb{R}^n\) and a permutation \(\sigma \in \mathfrak{S}_n\), \(\boldsymbol{t}_\sigma\) denotes \((t_{\sigma(1)},...,t_{\sigma(n)})\).

Call a multivariate function \(g: \mathbb{R}^n \to \mathbb{R}\) permutationinvariant if for any permutation \(\sigma\), it holds \(g(\boldsymbol{t}) = g(\boldsymbol{t}_\sigma)\) for all \(\boldsymbol{t}\in \mathbb{R}^n\).

For any function \(f: \mathbb{R}\to \mathbb{R}\), denote \(f^{\otimes n}: \left[ \mathbb{R}^n \to \mathbb{R}, \boldsymbol{t}\mapsto f(t_1)...f(t_n) \right]\). Call \(f^{\otimes n}\) the associated symmetric tensor function^{1} – tensor because it is a product of singlevariable functions, and symmetric because all of those singlevariable functions are the same.

For any multivariate function \(g: \mathbb{R}^n \to \mathbb{R}\), denote \(\mathop{\mathrm{Sym}}g: \left[ \mathbb{R}^n \to \mathbb{R}, \boldsymbol{t}\mapsto \frac{1}{n!} \sum_{\sigma \in \mathfrak{S}_n} g(\boldsymbol{t}_\sigma) \right]\).
We will sometimes write physiciststyle \(f(t)\) to mean a function \(f: \mathbb{R}\to \mathbb{R}\), and similarly \(g(\boldsymbol{t})\) instead of \(g: \mathbb{R}^n \to \mathbb{R}\).
Some function spaces Fix \(1 \leq p < \infty\) and \(q\) its conjugate exponent, i.e \(1/p+1/q=1\).

Let \(L^p(\mathbb{R})\) be the Banach space of \(L^p\)integrable functions over \(\mathbb{R}\) (with the usual Lebesgue measure). Its dual space is \(L^q(\mathbb{R})\).

Let \(C_0(\mathbb{R})\) be the space of vanishing continuous functions over \(\mathbb{R}\). Its dual space is \(\mathcal{M}(\mathbb{R})\), the space of Radon measures.^{2}

Similarly define \(L^p(\mathbb{R}^n)\), \(L^q(\mathbb{R}^n)\), \(C_0(\mathbb{R}^n)\) spaces of multivariate functions with \(n\) scalar variables.

Denote \(L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\), \(L^q_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\), \(C_{0 \mathop{\mathrm{Sym}}}(\mathbb{R}^n)\) the respective (closed) subspaces consisting of permutationinvariant functions.
Note that our shorthand \(\mathop{\mathrm{Sym}}\) can be viewed as a projection operator from \(L^p(\mathbb{R}^n)\) to \(L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\), and from \(C_0(\mathbb{R}^n)\) to \(C_{0 \mathop{\mathrm{Sym}}}(\mathbb{R}^n)\).
The result and why it looks surprising to me
Let \(1 \leq p < \infty\). The set \(\left\lbrace f^{\otimes n}(\boldsymbol{t}) ;~ f \in L^p(\mathbb{R}) \right\rbrace\) has its linear span dense in \(L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\).
The set \(\left\lbrace f^{\otimes n}(\boldsymbol{t}) ;~ f \in C_0(\mathbb{R}) \right\rbrace\) has its linear span dense in \(C_{0 \mathop{\mathrm{Sym}}}(\mathbb{R}^n)\).^{3}
More explicitly: any \(g(\boldsymbol{t}) \in C_{0 \mathop{\mathrm{Sym}}}(\mathbb{R}^n)\) is arbitrarilywell uniformly approximated by finite sums of the form \(\sum_{i \leq m} f_i(t_1)...f_i(t_n)\) (\(m < \infty\), \(f_i \in C_0(\mathbb{R})\)).
This is not Weierstrass with symmetrization As an obvious corollary, the proposition holds when \(\mathbb{R}\) is replaced by a closed interval \(I \subset \mathbb{R}\). In this case the result looks like a straightforward consequence of the Weierstrass approximation theorem, but it is not. Consider the following valid reasoning:
Fix a continuous function \(g(\boldsymbol{t})\) over the compact \(I^n\) and let \(\varepsilon>0\). By the Weierstrass approximation theorem, there exists a polynomial \(P(\boldsymbol{t})\) such that \(\left\lVert gP \right\rVert := \sup_{I^n} \left\lvert gP \right\rvert \leq \varepsilon\), and \(P(\boldsymbol{t})\) can be written as \(P(\boldsymbol{t}) = \sum_{\alpha \in \mathbb{N}^n} a_\alpha \boldsymbol{t}^\alpha\) (where there are only a finite number of nonzero coefficients \(a_\alpha\) and the shorthand \(\boldsymbol{t}^\alpha\) denotes \(t_1^{\alpha_1} ... t_n^{\alpha_n}\)).
If in addition \(g(\boldsymbol{t})\) is permutationinvariant, then the lemma below shows that \(\mathop{\mathrm{Sym}}P(\boldsymbol{t})\) is also an \(\varepsilon\)approximation of \(g(\boldsymbol{t})\), and it can be written as \(\mathop{\mathrm{Sym}}P(\boldsymbol{t}) = \sum_{\alpha \in \mathbb{N}^n} a_\alpha \mathop{\mathrm{Sym}}\boldsymbol{t}^\alpha = \sum_{\alpha \in \mathbb{N}^n} \sum_{\sigma \in \mathfrak{S}_n} \frac{a_\alpha}{n!} t_1^{\alpha_{\sigma(1)}} ... t_n^{\alpha_{\sigma(n)}}.\)
This does not show that \(g(\boldsymbol{t})\) can be approximated by a finite combination of symmetric tensor functions, as the reasoning may yield approximators such as \(t_1 t_2^3 + t_1^3 t_2\) (if \(n=2\)), which are not of the required form.
For any function \(g(\boldsymbol{t})\) over \(\mathbb{R}^n\) and any \(1 \leq p \leq \infty\), it holds: \(\left\lVert \mathop{\mathrm{Sym}}g \right\rVert_{L^p} \leq \left\lVert g \right\rVert_{L^p}\).
For any permutationinvariant \(g(\boldsymbol{t})\) and any function \(h(\boldsymbol{t})\), if \(\left\lVert g  h \right\rVert_{L^p} \leq \varepsilon\), then \(\left\lVert g  \mathop{\mathrm{Sym}}h \right\rVert_{L^p} \leq \varepsilon\).
Let any function \(g(\boldsymbol{t})\) over \(\mathbb{R}^n\) and any \(1 \leq p \leq \infty\). By definition of the \(L^p\) norm,
\[\left\lVert \mathop{\mathrm{Sym}}g \right\rVert_{L^p} = \left\lVert \frac{1}{n!} \sum_{\sigma \in \mathfrak{S}_n} g(\boldsymbol{t}_\sigma) \right\rVert_{L^p} \leq \frac{1}{n!} \sum_{\sigma \in \mathfrak{S}_n} \left\lVert g(\boldsymbol{t}_\sigma) \right\rVert_{L^p} = \left\lVert g \right\rVert_{L^p}.\]Let any permutationinvariant function \(g(\boldsymbol{t})\) and any function \(h(\boldsymbol{t})\) such that \(\left\lVert g  h \right\rVert_{L^p} \leq \varepsilon\). Then
\(\left\lVert g  \mathop{\mathrm{Sym}}h \right\rVert_{L^p} = \left\lVert \mathop{\mathrm{Sym}}(gh) \right\rVert_{L^p} \leq \left\lVert gh \right\rVert_{L^p} \leq \varepsilon.\)
An example of surprise In fact the case of permutationinvariant polynomials over a compact set is already surprising to me... In fact just the following example is already surprising to me:
Consider the function \(g(\boldsymbol{t}) = t_1 + ... + t_n\) over \([0,1]^n\). According to the proposition, there exist \(m<\infty\) and \(f_i(t) \in C([0,1])\) (\(i \leq m\)) such that \(g(\boldsymbol{t}) \approx \sum_{i \leq m} f_i(t_1)...f_i(t_n)\), in the sense of uniform approximation over \([0,1]^n\).
I wonder what these \(f_i\) could look like. Since they are continuous over \([0,1]\), according to the Weierstrass approximation theorem we may assume without loss of generality that each \(f_i\) is polynomial. Then, developing the product \(f_i(t_1)...f_i(t_n)\) would yield an a priori big polynomial, whereas directly using Weierstrass with symmetrization can yield simply \(t_1+...+t_n\) itself.
Brief proof of the result
In this section we prove the \(L^p/L^q\) (\(1 \leq p < \infty\)) part of the proposition; the \(C_0/\mathcal{M}\) part can be proved by the same arguments, with minor modifications.
I assume the reader is familiar with the basics of functional analysis and duality in Banach spaces. Recall the following density criterion, which is a consequence of the HahnBanach theorem (as are many things):
Let \(E\) be a Banach space and \(A\) a subset. \(\begin{aligned} \mathop{\mathrm{span}}(A) \text{ is dense in } E && \iff && \left\lbrace X \in E';~ \forall a \in A, \left\langle a, X \right\rangle_E = 0 \right\rbrace = \{ 0_{E'} \}. \end{aligned}\)
The set on the right is sometimes denoted \(A^\perp\) and called the annihilator of \(A\). Note that \(A^\perp = (\mathop{\mathrm{span}}(A))^\perp\). In the case where \(E\) is a Euclidean space, \(A^\perp\) is just (up to isometry) the orthogonal complement of \(\mathop{\mathrm{span}}(A)\).
The proposition will be proved by applying the above density criterion. To do so we will need the following intuitively obvious lemma, characterizing the dual of \(L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\). A formal and rather uninteresting proof can be found in appendix of the LaTeX version.
The dual of \(L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\) is isometrically isomorphic to \(L^q_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\).
We can now prove the proposition. The main argument is extracted from (Boyd Chua Desoer 1984, Theorem 2.5.2) in the context of Volterra series.
To apply the density criterion to \(A = \left\lbrace f^{\otimes n}(\boldsymbol{t}) ;~ f \in L^p(\mathbb{R}) \right\rbrace\) and \(E = L^p_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\), let \(h \in E' \simeq L^q_{\mathop{\mathrm{Sym}}}(\mathbb{R}^n)\) such that \(\left\langle f^{\otimes n}, h \right\rangle_{L^p} = 0\) for all \(f \in L^p(\mathbb{R})\). Let us show that \(h=0\), from which the proposition will follow.
Denote \(\Phi_h: L^p(\mathbb{R}) \to \mathbb{R}\) the \(n\)homogeneous map
\[\Phi_h[f] = \left\langle f^{\otimes n}, h \right\rangle_{L^p} = \int_{\mathbb{R}^n} d\boldsymbol{t}~ h(t_1,...,t_n) f(t_1)...f(t_n)\]and \(\Psi_h: L^p(\mathbb{R})^n \to \mathbb{R}\) the associated \(n\)linear system
\[\Psi_h\{f_1,...,f_n\} = \left\langle f_1 \otimes ... \otimes f_n, h \right\rangle_{L^p} = \int_{\mathbb{R}^n} d\boldsymbol{t}~ h(t_1,...,t_n) f_1(t_1)...f_n(t_n).\]The \(n\)linear system \(\Psi_h\{\cdot,...,\cdot\}\) is symmetric in its arguments, since \(h\) is permutationinvariant. So \(\Psi_h\) is completely determined by the \(n\)homogeneous map \(\Phi_h[\cdot]\) via the algebraic polarization identity
\[n! \Psi_h\{f_1,...,f_n\} = \left. \frac{\partial}{\partial \alpha_1 ... \partial \alpha_n} \right_{\boldsymbol{\alpha}=0} \Phi_h \left[ \sum_{i=1}^n \alpha_i f_i \right],\]and the righthandside is the differential of an identically zero map. Consequently,
\[\forall f_1,...,f_n \in L^p(\mathbb{R}),~ \Psi_h\{f_1,...,f_n\} = 0.\]Now evaluate this at \(f_1(t) = \boldsymbol{1}_{t \in A_1}, ..., f_n(t) = \boldsymbol{1}_{t \in A_n}\) for intervals \(A_i \subset \mathbb{R}\):
\[\Psi_h\{f_1,...,f_n\} = \int_{\mathbb{R}^n} d\boldsymbol{t}~ h(t_1,...,t_n) \boldsymbol{1}_{\boldsymbol{t}\in A_1 \times ... \times A_n} = 0.\]Since this holds for all \(A_i\), and hyperrectangles generate the Borel \(\sigma\)algebra, then \(h=0\), as claimed.
Is the result interesting/useful?
For the subjects that I’m currently leaning towards, the result presented in this document is actually pretty useless, as it only talks about approximability per se. It doesn’t give any guarantees on the nature nor the number of functions \(f_i\) required to \(\varepsilon\)approximate a given target function \(g\).
However I still find the result technically interesting and surprising. I never heard about it before but I’m certain it must be somewhere out there already – I would be glad to know where and in what context.

Disclaimer: the term "symmetric tensor function" may not be consistent with standard terminology, I haven’t checked. ↩

https://regularize.wordpress.com/2011/11/11/dualspacesofcontinuousfunctions/ ↩

I’m pretty sure the same holds if \(C_0\) is replaced by \(C_b\) i.e if we consider bounded continuous functions, instead of vanishing continuous. ↩