Distributions

The cube measures \(\delta(x; c)\) E0016 are finitely supported signed measures. The point distributions \(\delta(x; \xi)\) are their smooth counterparts, supported at a single point and parameterized by symmetric probes E0027.

(1) Definition (Smooth functions and distributions).

  • \(\mathcal{E}(X) := C^\infty(X, \IR)\) with the standard Fréchet topology (for \(X\) finite-dimensional).
  • \(\mathcal{E}'(X)\) is the space of compactly supported distributions (continuous dual of \(\mathcal{E}(X)\)).
  • \(\mathcal{E}'(X; x)\) is the subspace of distributions supported at \(x\).
  • \(\mathcal{E}'(X; x)_{\leq k}\) is its subspace of order \(\leq k\).

(2) Definition (Point distributions).

  • For \(\xi \in ST_k(X; x)\), the point distribution \(\delta(x; \xi)\) is

    \[ \langle \delta(x; \xi), f \rangle := D(f; x; \xi), \qquad f \in C^k(X, \IR). \]

    In particular \(\delta(x; 1) = \delta(x)\) is evaluation at \(x\).

  • The map \(\xi \mapsto \delta(x; \xi)\) is a linear isomorphism \(ST_k(X; x) \xrightarrow{\sim} \mathcal{E}'(X; x)_{\leq k}\) (Schwartz structure theorem).

(3) Proposition (Measures embed in distributions). Restriction along \(\mathcal{E}(X) \subseteq F(X)\) embeds the finitely supported measures into the distributions as the order-\(0\) distributions with finite support. In particular, every cube measure \(\delta(x; c)\) E0016 is a distribution.

Proof. Evaluation \(f \mapsto f(z)\) at a point \(z\) is continuous on \(\mathcal{E}(X)\) and extends to \(C^0(X, \IR)\), so a finitely supported measure \(\mu = \sum_i \lambda_i \delta(z_i)\) defines a distribution of order \(0\). The map is injective since the \(\delta(z_i)\) are linearly independent as functionals on \(\mathcal{E}(X)\).

(4) Theorem (Distribution pushforward). For \(\phi\) \(C^k\) near \(x\) and \(\xi \in ST_k(X; x)\),

\[ \phi_* \delta(x; \xi) = \delta(y; D_+(\phi; x)\, \xi). \]

Proof. \(\langle \phi_* \delta(x; \xi), f \rangle = D(\phi^* f; x; \xi) = D(f; y; D_+(\phi; x)\, \xi) = \langle \delta(y; D_+(\phi; x)\, \xi), f \rangle\) by the smooth adjunction E0028.

(5) Proposition (Filtration and principal symbol). \(D_+(\phi; x)\) preserves the degree filtration, and on the associated graded it is the symmetric power of the first derivative:

\[ \gr_r\, D_+(\phi; x) = \SYM^r(D(\phi; x)) : \SYM^r(E) \to \SYM^r(F). \]

Proof. A partition \(\pi \in \Part(r)\) contributes in degree \(|\pi| \leq r\), with equality only for the partition into singletons, whose term is \(\prod_i D(\phi; x; v_i)\).


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