Abstract
In the present study a specific kind of widespread mathematical objects: descriptor matrices are defined and studied. These are matrices connected with several problems concerning many fields of interest in theoretical chemistry, classical and quantum mechanics, or quantitative structure-properties relations. The twofold dimensionality structure of descriptor matrices is analyzed and the properties of descriptor matrices are also disclosed with respect origin shifts and rotations. A tensor to study schematically the three dimensional nature of many particle structures, the characteristic form tensor is defined. The construction of similarity matrices from descriptor matrices and the connection with quantum similarity are finally discussed.
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Notes
In fact, there must be also considered present an alternative parallel description in dual space, involving the transposes of all vectors and matrices which are employed in this work. The twofold dimensions appearing in the main discussion will be therefore reversed looking at this dual case. However, such issue will not be discussed here, in order to keep the formalism as simple as possible.
Here the Dirac’s notation is used for column vectors, which will be written using a ket symbol: \(\left| \mathbf{a} \right\rangle \), while row vectors will be noted by a bra symbol \(\left\langle \mathbf{a} \right| \). Both symbols correspond to the transpose one from the other. As the field of reference along this study will be the real field, then there is no conjugation involved.
The symbol \({*}\) involving two vectors of a given vector space: \(\forall \left| \mathbf{a} \right\rangle ,\left| \mathbf{b} \right\rangle \in \text{ V }_D :\left| \mathbf{c} \right\rangle =\left| \mathbf{a} \right\rangle {*}\left| \mathbf{b} \right\rangle \rightarrow \forall I:c_{I} =a_{I} b_{I} \rightarrow \left| \mathbf{c} \right\rangle \in \text{ V }_{D} \) denotes an inward product, acting on two vectors and yielding another vector of the same vector space. The symbol involving a vector: \(\alpha =\left\langle {\left| \mathbf{a} \right\rangle } \right\rangle =\sum _I {a_{I}} \) corresponds to the complete sum of its elements. Therefore, the expression: \(\left\langle {\left| \mathbf{a} \right\rangle {*}\left| \mathbf{b} \right\rangle } \right\rangle =\sum _I {a_{I} b_{I} } \equiv \left\langle \mathbf{a} | \mathbf{b} \right\rangle \) corresponds to the scalar product of the two vectors. In the previous definitions have been used column vectors but the same definitions hold for row vectors, matrices or hypermatrices. If the involved vectors are functions: \(\left| f \right\rangle \equiv f\left( \mathbf{r} \right) \), the inward products \(\left| f \right\rangle {*}\left| g \right\rangle \equiv f\left( \mathbf{r} \right) g\left( \mathbf{r} \right) \) are coincident with products of functions, and the complete sum of a function becomes an integral over the definition domain:\(\left\langle {f\left( \mathbf{r} \right) } \right\rangle =\int _D {f\left( \mathbf{r} \right) d\mathbf{r}.} \) A scalar product of two functions can be written in this notation as: \(\left\langle {\left| f \right\rangle {*}\left| g \right\rangle } \right\rangle =\int _{D} {f\left( \mathbf{r} \right) g\left( \mathbf{r} \right) d\mathbf{r}\equiv \left\langle f | g \right\rangle }.\)
Provided that the symbol \(\left[ \!\left[ \mathbf{A} \right] \!\right] \), applied over a matrix with functions as elements: \(\mathbf{A}\left( \mathbf{r} \right) =\left\{ {A_{IJ} \left( \mathbf{r} \right) } \right\} \), might be considered that yields the matrix of the integrals of the function elements: \(\mathbf{G}=\left[ \!\left[ \mathbf{A} \right] \!\right] =\left\{ {G_{IJ} =\int _D {A_{IJ} \left( \mathbf{r} \right) d\mathbf{r}} } \right\} .\)
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Carbó-Dorca, R. Particle coordinates and discrete molecular description: a geometric point of view on a twofold dimensionality environment. J Math Chem 51, 1569–1583 (2013). https://doi.org/10.1007/s10910-013-0165-4
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DOI: https://doi.org/10.1007/s10910-013-0165-4