Khatri–Rao product

In mathematics, the Khatri–Rao product or block Kronecker product of two partitioned matrices ${\displaystyle \mathbf {A} }$ and ${\displaystyle \mathbf {B} }$ is defined as^[1][2][3]

${\displaystyle \mathbf {A} \ast \mathbf {B} =\left(\mathbf {A} _{ij}\otimes \mathbf {B} _{ij}\right)_{ij}}$

in which the ij-th block is the m_ip_i × n_jq_j sized Kronecker product of the corresponding blocks of A and B, assuming the number of row and column partitions of both matrices is equal. The size of the product is then (Σ_i m_ip_i) × (Σ_j n_jq_j).

For example, if A and B both are 2 × 2 partitioned matrices e.g.:

${\displaystyle \mathbf {A} =\left[{\begin{array}{c | c}\mathbf {A} _{11}&\mathbf {A} _{12}\\\hline \mathbf {A} _{21}&\mathbf {A} _{22}\end{array}}\right]=\left[{\begin{array}{c c | c}1&2&3\\4&5&6\\\hline 7&8&9\end{array}}\right],\quad \mathbf {B} =\left[{\begin{array}{c | c}\mathbf {B} _{11}&\mathbf {B} _{12}\\\hline \mathbf {B} _{21}&\mathbf {B} _{22}\end{array}}\right]=\left[{\begin{array}{c | c c}1&4&7\\\hline 2&5&8\\3&6&9\end{array}}\right],}$

we obtain:

${\displaystyle \mathbf {A} \ast \mathbf {B} =\left[{\begin{array}{c | c}\mathbf {A} _{11}\otimes \mathbf {B} _{11}&\mathbf {A} _{12}\otimes \mathbf {B} _{12}\\\hline \mathbf {A} _{21}\otimes \mathbf {B} _{21}&\mathbf {A} _{22}\otimes \mathbf {B} _{22}\end{array}}\right]=\left[{\begin{array}{c c | c c}1&2&12&21\\4&5&24&42\\\hline 14&16&45&72\\21&24&54&81\end{array}}\right].}$

This is a submatrix of the Tracy–Singh product ^[4] of the two matrices (each partition in this example is a partition in a corner of the Tracy–Singh product).

Column-wise Kronecker product

The column-wise Kronecker product of two matrices is a special case of the Khatri-Rao product as defined above, and may also be called the Khatri–Rao product. This product assumes the partitions of the matrices are their columns. In this case m₁ = m, p₁ = p, n = q and for each j: n_j = q_j = 1. The resulting product is a mp × n matrix of which each column is the Kronecker product of the corresponding columns of A and B. Using the matrices from the previous examples with the columns partitioned:

${\displaystyle \mathbf {C} =\left[{\begin{array}{c | c | c}\mathbf {C} _{1}&\mathbf {C} _{2}&\mathbf {C} _{3}\end{array}}\right]=\left[{\begin{array}{c | c | c}1&2&3\\4&5&6\\7&8&9\end{array}}\right],\quad \mathbf {D} =\left[{\begin{array}{c | c | c }\mathbf {D} _{1}&\mathbf {D} _{2}&\mathbf {D} _{3}\end{array}}\right]=\left[{\begin{array}{c | c | c }1&4&7\\2&5&8\\3&6&9\end{array}}\right],}$

so that:

${\displaystyle \mathbf {C} \ast \mathbf {D} =\left[{\begin{array}{c | c | c }\mathbf {C} _{1}\otimes \mathbf {D} _{1}&\mathbf {C} _{2}\otimes \mathbf {D} _{2}&\mathbf {C} _{3}\otimes \mathbf {D} _{3}\end{array}}\right]=\left[{\begin{array}{c | c | c }1&8&21\\2&10&24\\3&12&27\\4&20&42\\8&25&48\\12&30&54\\7&32&63\\14&40&72\\21&48&81\end{array}}\right].}$

This column-wise version of the Khatri–Rao product is useful in linear algebra approaches to data analytical processing^[5] and in optimizing the solution of inverse problems dealing with a diagonal matrix.^[6][7]

In 1996 the column-wise Khatri–Rao product was proposed to estimate the angles of arrival (AOAs) and delays of multipath signals^[8] and four coordinates of signals sources^[9] at a digital antenna array.

Face-splitting product

Face splitting product of matrices

An alternative concept of the matrix product, which uses row-wise splitting of matrices with a given quantity of rows, was proposed by V. Slyusar^[10] in 1996.^{[9][11][12][13][14]}

This matrix operation was named the "face-splitting product" of matrices^[11][13] or the "transposed Khatri–Rao product". This type of operation is based on row-by-row Kronecker products of two matrices. Using the matrices from the previous examples with the rows partitioned:

${\displaystyle \mathbf {C} ={\begin{bmatrix}\mathbf {C} _{1}\\\hline \mathbf {C} _{2}\\\hline \mathbf {C} _{3}\\\end{bmatrix}}={\begin{bmatrix}1&2&3\\\hline 4&5&6\\\hline 7&8&9\end{bmatrix}},\quad \mathbf {D} ={\begin{bmatrix}\mathbf {D} _{1}\\\hline \mathbf {D} _{2}\\\hline \mathbf {D} _{3}\\\end{bmatrix}}={\begin{bmatrix}1&4&7\\\hline 2&5&8\\\hline 3&6&9\end{bmatrix}},}$

the result can be obtained:^[9][11][13]

${\displaystyle \mathbf {C} \bullet \mathbf {D} ={\begin{bmatrix}\mathbf {C} _{1}\otimes \mathbf {D} _{1}\\\hline \mathbf {C} _{2}\otimes \mathbf {D} _{2}\\\hline \mathbf {C} _{3}\otimes \mathbf {D} _{3}\\\end{bmatrix}}={\begin{bmatrix}1&4&7&2&8&14&3&12&21\\\hline 8&20&32&10&25&40&12&30&48\\\hline 21&42&63&24&48&72&27&54&81\end{bmatrix}}.}$

Main properties

In the following properties, the operator ${\textstyle \bullet }$ denotes the row-wise Kronecker product (face-splitting product) and the operator ${\textstyle \ast }$ denotes the column-wise Kronecker product

1. Transpose (V. Slyusar, 1996^[9][11][12]):

   ${\displaystyle \left(\mathbf {A} \bullet \mathbf {B} \right)^{\textsf {T}}={\textbf {A}}^{\textsf {T}}\ast \mathbf {B} ^{\textsf {T}}}$,

2. Bilinearity and associativity:^[9][11][12]

   ${\displaystyle {\begin{aligned}\mathbf {A} \bullet (\mathbf {B} +\mathbf {C} )&=\mathbf {A} \bullet \mathbf {B} +\mathbf {A} \bullet \mathbf {C} ,\\(\mathbf {B} +\mathbf {C} )\bullet \mathbf {A} &=\mathbf {B} \bullet \mathbf {A} +\mathbf {C} \bullet \mathbf {A} ,\\(k\mathbf {A} )\bullet \mathbf {B} &=\mathbf {A} \bullet (k\mathbf {B} )=k(\mathbf {A} \bullet \mathbf {B} ),\\(\mathbf {A} \bullet \mathbf {B} )\bullet \mathbf {C} &=\mathbf {A} \bullet (\mathbf {B} \bullet \mathbf {C} ),\\\end{aligned}}}$

   where A, B and C are matrices, and k is a scalar,

   ${\displaystyle a\bullet \mathbf {B} =\mathbf {B} \bullet a}$,^[12]

   where ${\displaystyle a}$ is a vector,
3. The mixed-product property (V. Slyusar, 1997^[12]):

   ${\displaystyle (\mathbf {A} \bullet \mathbf {B} )\left(\mathbf {A} ^{\textsf {T}}\ast \mathbf {B} ^{\textsf {T}}\right)=\left(\mathbf {A} \mathbf {A} ^{\textsf {T}}\right)\circ \left(\mathbf {B} \mathbf {B} ^{\textsf {T}}\right)}$,

   ${\displaystyle (\mathbf {A} \bullet \mathbf {B} )(\mathbf {C} \ast \mathbf {D} )=(\mathbf {A} \mathbf {C} )\circ (\mathbf {B} \mathbf {D} )}$,^[13]

   ${\displaystyle (\mathbf {A} \bullet \mathbf {B} \bullet \mathbf {C} \bullet \mathbf {D} )(\mathbf {L} \ast \mathbf {M} \ast \mathbf {N} \ast \mathbf {P} )=(\mathbf {A} \mathbf {L} )\circ (\mathbf {B} \mathbf {M} )\circ (\mathbf {C} \mathbf {N} )\circ (\mathbf {D} \mathbf {P} )}$^[15]

   ${\displaystyle (\mathbf {A} \ast \mathbf {B} )^{\textsf {T}}(\mathbf {A} \ast \mathbf {B} )=\left(\mathbf {A} ^{\textsf {T}}\mathbf {A} \right)\circ \left(\mathbf {B} ^{\textsf {T}}\mathbf {B} \right)}$,^[16]

   where ${\displaystyle \circ }$ denotes the Hadamard product,
4. ${\displaystyle (\mathbf {A} \circ \mathbf {B} )\bullet (\mathbf {C} \circ \mathbf {D} )=(\mathbf {A} \bullet \mathbf {C} )\circ (\mathbf {B} \bullet \mathbf {D} )}$,^[12]
5. ${\displaystyle \ a\otimes (\mathbf {B} \bullet \mathbf {C} )=(a\otimes \mathbf {B} )\bullet \mathbf {C} }$,^[9] where ${\displaystyle a}$ is a row vector,
6. ${\displaystyle (\mathbf {A} \otimes \mathbf {B} )(\mathbf {C} \ast \mathbf {D} )=(\mathbf {A} \mathbf {C} )\ast (\mathbf {B} \mathbf {D} )}$,^[16]
7. ${\displaystyle (\mathbf {A} \otimes \mathbf {B} )\ast (\mathbf {C} \otimes \mathbf {D} )=\mathbf {P} [(\mathbf {A} \ast \mathbf {C} )\otimes (\mathbf {B} \ast \mathbf {D} )]}$, where ${\displaystyle \mathbf {P} }$ is a permutation matrix.^[7]
8.  

   ${\displaystyle (\mathbf {A} \bullet \mathbf {B} )(\mathbf {C} \otimes \mathbf {D} )=(\mathbf {A} \mathbf {C} )\bullet (\mathbf {B} \mathbf {D} )}$,^[13][15]

   Similarly:

   ${\displaystyle (\mathbf {A} \bullet \mathbf {L} )(\mathbf {B} \otimes \mathbf {M} )\cdots (\mathbf {C} \otimes \mathbf {S} )=(\mathbf {A} \mathbf {B} \cdots \mathbf {C} )\bullet (\mathbf {L} \mathbf {M} \cdots \mathbf {S} )}$,

9.  

   ${\displaystyle c^{\textsf {T}}\bullet d^{\textsf {T}}=c^{\textsf {T}}\otimes d^{\textsf {T}}}$,^[12]

   ${\displaystyle c\ast d=c\otimes d}$,

   where ${\displaystyle c}$ and ${\displaystyle d}$ are vectors,
10. ${\displaystyle \left(\mathbf {A} \ast c^{\textsf {T}}\right)d=\left(\mathbf {A} \ast d^{\textsf {T}}\right)c}$,^[17] ${\displaystyle d^{\textsf {T}}\left(c\bullet \mathbf {A} ^{\textsf {T}}\right)=c^{\textsf {T}}\left(d\bullet \mathbf {A} ^{\textsf {T}}\right)}$,
11.  

    ${\displaystyle (\mathbf {A} \bullet \mathbf {B} )(c\otimes d)=(\mathbf {A} c)\circ (\mathbf {B} d)}$,^[18]

    where ${\displaystyle c}$ and ${\displaystyle d}$ are vectors (it is a combine of properties 3 an 8),

    Similarly:

    ${\displaystyle (\mathbf {A} \bullet \mathbf {B} )(\mathbf {M} \mathbf {N} c\otimes \mathbf {Q} \mathbf {P} d)=(\mathbf {A} \mathbf {M} \mathbf {N} c)\circ (\mathbf {B} \mathbf {Q} \mathbf {P} d),}$

12.  

    ${\displaystyle {\mathcal {F}}\left((C^{(1)}x)\star (C^{(2)}y)\right)=\left(({\mathcal {F}}C^{(1)})\bullet ({\mathcal {F}}C^{(2)})\right)(x\otimes y)=({\mathcal {F}}C^{(1)}x)\circ ({\mathcal {F}}C^{(2)}y)}$,

    where ${\displaystyle \star }$ is vector convolution; ${\displaystyle C^{(1)},C^{(2)}}$ are "count sketch" matrices; and ${\displaystyle {\mathcal {F}}}$ is the Fourier transform matrix (this result is an evolving of count sketch properties^[19]).

    
    
    This can be generalized for appropriate matrices ${\displaystyle \mathbf {A} ,\mathbf {B} }$:

    ${\displaystyle {\mathcal {F}}\left((\mathbf {A} x)\star (\mathbf {B} y)\right)=\left(({\mathcal {F}}\mathbf {A} )\bullet ({\mathcal {F}}\mathbf {B} )\right)(x\otimes y)=({\mathcal {F}}\mathbf {A} x)\circ ({\mathcal {F}}\mathbf {B} y)}$

    because property 11 above gives us

    ${\displaystyle \left(({\mathcal {F}}\mathbf {A} )\bullet ({\mathcal {F}}\mathbf {B} )\right)(x\otimes y)=({\mathcal {F}}\mathbf {A} x)\circ ({\mathcal {F}}\mathbf {B} y)}$

    And the convolution theorem gives us

    ${\displaystyle {\mathcal {F}}\left((\mathbf {A} x)\star (\mathbf {B} y)\right)=({\mathcal {F}}\mathbf {A} x)\circ ({\mathcal {F}}\mathbf {B} y)}$

13.  

    ${\displaystyle \mathbf {A} \bullet \mathbf {B} =\left(\mathbf {A} \otimes \mathbf {1_{k}} ^{\textsf {T}}\right)\circ \left(\mathbf {1_{c}} ^{\textsf {T}}\otimes \mathbf {B} \right)}$,^[20]

    where ${\displaystyle \mathbf {A} }$ is ${\displaystyle r\times c}$ matrix, ${\displaystyle \mathbf {B} }$ is ${\displaystyle r\times k}$ matrix, ${\displaystyle \mathbf {1_{c}} }$ is a vector of 1's of length ${\displaystyle c}$, and ${\displaystyle \mathbf {1_{k}} }$ is a vector of 1's of length ${\displaystyle k}$

    or

    ${\displaystyle \mathbf {M} \bullet \mathbf {M} =\left(\mathbf {M} \otimes \mathbf {1} ^{\textsf {T}}\right)\circ \left(\mathbf {1} ^{\textsf {T}}\otimes \mathbf {M} \right)}$,^[21]

    where ${\displaystyle \mathbf {M} }$ is ${\displaystyle r\times c}$ matrix, ${\displaystyle \circ }$ means element by element multiplication and ${\displaystyle \mathbf {1} }$ is a vector of 1's of length ${\displaystyle c}$.

    ${\displaystyle \mathbf {M} \bullet \mathbf {M} =\mathbf {M} [\circ ]\left(\mathbf {M} \otimes \mathbf {1} ^{\textsf {T}}\right)}$,

    where ${\displaystyle [\circ ]}$ denotes the penetrating face product of matrices.^[13]

    Similarly:

    ${\displaystyle \mathbf {P} \ast \mathbf {N} =(\mathbf {P} \otimes \mathbf {1_{k}} )\circ (\mathbf {1_{c}} \otimes \mathbf {N} )}$, where ${\displaystyle \mathbf {P} }$ is ${\displaystyle c\times r}$ matrix, ${\displaystyle \mathbf {N} }$ is ${\displaystyle k\times r}$ matrix,.

14.  

    ${\displaystyle \mathbf {W_{d}} \mathbf {A} =\mathbf {w} \bullet \mathbf {A} }$,^[12]

    ${\displaystyle \operatorname {vec} ((\mathbf {w} ^{\textsf {T}}\ast \mathbf {A} )\mathbf {B} )=(\mathbf {B} ^{\textsf {T}}\ast \mathbf {A} )\mathbf {w} }$^[13]= ${\displaystyle \operatorname {vec} (\mathbf {A} (\mathbf {w} \bullet \mathbf {B} ))}$= ${\displaystyle \operatorname {vec} (\mathbf {A} \mathbf {W_{d}} \mathbf {B} )}$,
    

    ${\displaystyle \operatorname {vec} \left(\mathbf {A} ^{\textsf {T}}\mathbf {W_{d}} \mathbf {A} \right)=\left(\mathbf {A} \bullet \mathbf {A} \right)^{\textsf {T}}\mathbf {w} }$,^[21]

    where ${\displaystyle \mathbf {w} }$ is the vector consisting of the diagonal elements of ${\displaystyle \mathbf {W_{d}} }$, ${\displaystyle \operatorname {vec} (\mathbf {A} )}$ means stack the columns of a matrix ${\displaystyle \mathbf {A} }$ on top of each other to give a vector.
15.  

    ${\displaystyle (\mathbf {A} \bullet \mathbf {L} )(\mathbf {B} \otimes \mathbf {M} )\cdots (\mathbf {C} \otimes \mathbf {S} )(\mathbf {K} \ast \mathbf {T} )=(\mathbf {A} \mathbf {B} ...\mathbf {C} \mathbf {K} )\circ (\mathbf {L} \mathbf {M} ...\mathbf {S} \mathbf {T} )}$.^[13][15]

    Similarly:

    ${\displaystyle {\begin{aligned}(\mathbf {A} \bullet \mathbf {L} )(\mathbf {B} \otimes \mathbf {M} )\cdots (\mathbf {C} \otimes \mathbf {S} )(c\otimes d)&=(\mathbf {A} \mathbf {B} \cdots \mathbf {C} c)\circ (\mathbf {L} \mathbf {M} \cdots \mathbf {S} d),\\(\mathbf {A} \bullet \mathbf {L} )(\mathbf {B} \otimes \mathbf {M} )\cdots (\mathbf {C} \otimes \mathbf {S} )(\mathbf {P} c\otimes \mathbf {Q} d)&=(\mathbf {A} \mathbf {B} \cdots \mathbf {C} \mathbf {P} c)\circ (\mathbf {L} \mathbf {M} \cdots \mathbf {S} \mathbf {Q} d)\end{aligned}}}$,

    where ${\displaystyle c}$ and ${\displaystyle d}$ are vectors
16.  

    If ${\textstyle \mathbf {B} }$ is a diagonal matrix and ${\textstyle \mathbf {b} }$ is its main diagonal:

    ${\displaystyle \operatorname {vec} (\mathbf {A} \mathbf {B} \mathbf {C} )=(\mathbf {C} ^{\mathrm {T} }\ast \mathbf {A} )\mathbf {b} }$

    Here, ${\textstyle \operatorname {vec} (\cdot )}$ is the column-wise vectorization operator.

Examples

Source:^[18]

${\displaystyle {\begin{aligned}&\left({\begin{bmatrix}1&0\\0&1\\1&0\end{bmatrix}}\bullet {\begin{bmatrix}1&0\\1&0\\0&1\end{bmatrix}}\right)\left({\begin{bmatrix}1&1\\1&-1\end{bmatrix}}\otimes {\begin{bmatrix}1&1\\1&-1\end{bmatrix}}\right)\left({\begin{bmatrix}\sigma _{1}&0\\0&\sigma _{2}\\\end{bmatrix}}\otimes {\begin{bmatrix}\rho _{1}&0\\0&\rho _{2}\\\end{bmatrix}}\right)\left({\begin{bmatrix}x_{1}\\x_{2}\end{bmatrix}}\ast {\begin{bmatrix}y_{1}\\y_{2}\end{bmatrix}}\right)\\[5pt]{}={}&\left({\begin{bmatrix}1&0\\0&1\\1&0\end{bmatrix}}\bullet {\begin{bmatrix}1&0\\1&0\\0&1\end{bmatrix}}\right)\left({\begin{bmatrix}1&1\\1&-1\end{bmatrix}}{\begin{bmatrix}\sigma _{1}&0\\0&\sigma _{2}\\\end{bmatrix}}{\begin{bmatrix}x_{1}\\x_{2}\end{bmatrix}}\,\otimes \,{\begin{bmatrix}1&1\\1&-1\end{bmatrix}}{\begin{bmatrix}\rho _{1}&0\\0&\rho _{2}\\\end{bmatrix}}{\begin{bmatrix}y_{1}\\y_{2}\end{bmatrix}}\right)\\[5pt]{}={}&{\begin{bmatrix}1&0\\0&1\\1&0\end{bmatrix}}{\begin{bmatrix}1&1\\1&-1\end{bmatrix}}{\begin{bmatrix}\sigma _{1}&0\\0&\sigma _{2}\\\end{bmatrix}}{\begin{bmatrix}x_{1}\\x_{2}\end{bmatrix}}\,\circ \,{\begin{bmatrix}1&0\\1&0\\0&1\end{bmatrix}}{\begin{bmatrix}1&1\\1&-1\end{bmatrix}}{\begin{bmatrix}\rho _{1}&0\\0&\rho _{2}\\\end{bmatrix}}{\begin{bmatrix}y_{1}\\y_{2}\end{bmatrix}}.\end{aligned}}}$

Theorem

Source:^[18]

If ${\displaystyle M=T^{(1)}\bullet \dots \bullet T^{(c)}}$, where ${\displaystyle T^{(1)},\dots ,T^{(c)}}$ are independent components a random matrix ${\displaystyle T}$ with independent identically distributed rows ${\displaystyle T_{1},\dots ,T_{m}\in \mathbb {R} ^{d}}$, such that

${\displaystyle E\left[(T_{1}x)^{2}\right]=\left\|x\right\|_{2}^{2}}$ and ${\displaystyle E\left[(T_{1}x)^{p}\right]^{\frac {1}{p}}\leq {\sqrt {ap}}\|x\|_{2}}$,

then for any vector ${\displaystyle x}$

${\displaystyle \left|\left\|Mx\right\|_{2}-\left\|x\right\|_{2}\right|<\varepsilon \left\|x\right\|_{2}}$

with probability ${\displaystyle 1-\delta }$ if the quantity of rows

${\displaystyle m=(4a)^{2c}\varepsilon ^{-2}\log 1/\delta +(2ae)\varepsilon ^{-1}(\log 1/\delta )^{c}.}$

In particular, if the entries of ${\displaystyle T}$ are ${\displaystyle \pm 1}$ can get

${\displaystyle m=O\left(\varepsilon ^{-2}\log 1/\delta +\varepsilon ^{-1}\left({\frac {1}{c}}\log 1/\delta \right)^{c}\right)}$

which matches the Johnson–Lindenstrauss lemma of ${\displaystyle m=O\left(\varepsilon ^{-2}\log 1/\delta \right)}$ when ${\displaystyle \varepsilon }$ is small.

Block face-splitting product

Transposed block face-splitting product in the context of a multi-face radar model^[15]

According to the definition of V. Slyusar^[9][13] the block face-splitting product of two partitioned matrices with a given quantity of rows in blocks

${\displaystyle \mathbf {A} =\left[{\begin{array}{c | c}\mathbf {A} _{11}&\mathbf {A} _{12}\\\hline \mathbf {A} _{21}&\mathbf {A} _{22}\end{array}}\right],\quad \mathbf {B} =\left[{\begin{array}{c | c}\mathbf {B} _{11}&\mathbf {B} _{12}\\\hline \mathbf {B} _{21}&\mathbf {B} _{22}\end{array}}\right],}$

can be written as :

${\displaystyle \mathbf {A} [\bullet ]\mathbf {B} =\left[{\begin{array}{c | c}\mathbf {A} _{11}\bullet \mathbf {B} _{11}&\mathbf {A} _{12}\bullet \mathbf {B} _{12}\\\hline \mathbf {A} _{21}\bullet \mathbf {B} _{21}&\mathbf {A} _{22}\bullet \mathbf {B} _{22}\end{array}}\right].}$

The transposed block face-splitting product (or Block column-wise version of the Khatri–Rao product) of two partitioned matrices with a given quantity of columns in blocks has a view:^[9][13]

${\displaystyle \mathbf {A} [\ast ]\mathbf {B} =\left[{\begin{array}{c | c}\mathbf {A} _{11}\ast \mathbf {B} _{11}&\mathbf {A} _{12}\ast \mathbf {B} _{12}\\\hline \mathbf {A} _{21}\ast \mathbf {B} _{21}&\mathbf {A} _{22}\ast \mathbf {B} _{22}\end{array}}\right].}$

Main properties

1. Transpose:

   ${\displaystyle \left(\mathbf {A} [\ast ]\mathbf {B} \right)^{\textsf {T}}={\textbf {A}}^{\textsf {T}}[\bullet ]\mathbf {B} ^{\textsf {T}}}$^[15]

Applications

The Face-splitting product and the Block Face-splitting product used in the tensor-matrix theory of digital antenna arrays. These operations are also used in:

• Artificial Intelligence and Machine learning systems to minimization of convolution and tensor sketch operations,^[18]
• A popular Natural Language Processing models, and hypergraph models of similarity,^[22]
• Generalized linear array model in statistics^[21]
• Two- and multidimensional P-spline approximation of data,^[20]
• Studies of genotype x environment interactions.^[23]

See also

• Kronecker product
• Hadamard product