Did some fixing. I hope now I have the signs right (still here):

$WF(K_1 \circ K_2) \;\subset\; \left\{ (x,z, k_x, k_z) \;\vert\; \array{ \left( (x,y,k_x,-k_y) \in WF(K_1) \,\, \text{and} \,\, (y,z,k_y, k_z) \in WF(K_2) \right) \\ \text{or} \\ \left( k_x = 0 \,\text{and}\, (y,z,0,-k_z) \in WF(K_2) \right) \\ \text{or} \\ \left( k_z = 0 \,\text{and}\, (x,y,k_x,0) \in WF(K_1) \right) } \right\}$ ]]>I have added the statement for partial product of distributions of several variables, and composition of integral kernels: here

]]>added statement and proof of the estimate of $WF(u\cdot v)$ (here)

]]>for ease of linking, I gave *Hörmander’s criterion* a little entry of its own.

I have given *product of distributions* an Idea-section (here) which means to explain, informally but accurately, how the product is defined via convolution of Fourier transforms “around any point” and how that immediately implies Hörmander’s condition on the wave front sets of the two factors.

I have spelled out the definition of the product of distributions with compatible wave front sets (from Hörmander 90) and the proof that this is well defined: here.

That proof however relies on a little hierarchy of lemmas. I have added statement (not the proof yet, but pointers to Hörmander’s proofs) of the next lower lemmas at *tensor product of distributions* and at *pullback of distributions*.

Since nobody objected to the suggestion #1, I went ahead and implemented it. Details on what I did are here.

]]>I gave *product of distributions* its own entry. For the moment it just points to the definition in Hörmander’s book.

This should eventually supercede the section “Multiplication of distributions” at *distributions*, which I find suboptimal: that section starts very vaguely referring to physics as if the issue only appears there, and it keeps being very vague, with its three sub-subsections being little more than a pointer to one reference by Colombeau.

I suggest to

remove that whole subsection at

*distribution*and leave just a pointer to*product of distributions*move the mentioning of Colombeau’s reference to

*product of distributions*and say how it relates to Hörmander’s definitionremove all vague mentioning of application in physics and instead add a pointer to

*Wick algebra*and*microcausal functional*, which I will create shortly.