Matricu reizināšana ir bināra operācija , kurā tiek reizināts matricu pāris, šīs operācijas rezultātā tiek iegūta jauna matrica. Skaitļi (piemēram, reāli , kompleksi skaitļi) var tikt reizināti kā elementārajā aritmētikā .
Pastāv vairāki veidi, kā reizināt matricas, no kuriem vienkāršākais ir reizināšana ar skaitli. Matricu reizināšana nav komutatīva , kas nozīmē, ka A · B ≠ B · A A n × m B m × p AB n × p
Vienkāršākā reizināšana ar matricām ir matricas reizināšana ar skaitli.
Skaitļa λ A λ A A λ A
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{\displaystyle (\lambda \mathbf {A} )_{ij}=\lambda \left(\mathbf {A} \right)_{ij}\,,}
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{\displaystyle \lambda \mathbf {A} =\lambda {\begin{pmatrix}A_{11}&A_{12}&\cdots &A_{1m}\\A_{21}&A_{22}&\cdots &A_{2m}\\\vdots &\vdots &\ddots &\vdots \\A_{n1}&A_{n2}&\cdots &A_{nm}\\\end{pmatrix}}={\begin{pmatrix}\lambda A_{11}&\lambda A_{12}&\cdots &\lambda A_{1m}\\\lambda A_{21}&\lambda A_{22}&\cdots &\lambda A_{2m}\\\vdots &\vdots &\ddots &\vdots \\\lambda A_{n1}&\lambda A_{n2}&\cdots &\lambda A_{nm}\\\end{pmatrix}}\,.}
Līdzīgi tiek definēts matricas A λ
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{\displaystyle (\mathbf {A} \lambda )_{ij}=\left(\mathbf {A} \right)_{ij}\lambda \,,}
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{\displaystyle \mathbf {A} \lambda ={\begin{pmatrix}A_{11}&A_{12}&\cdots &A_{1m}\\A_{21}&A_{22}&\cdots &A_{2m}\\\vdots &\vdots &\ddots &\vdots \\A_{n1}&A_{n2}&\cdots &A_{nm}\\\end{pmatrix}}\lambda ={\begin{pmatrix}A_{11}\lambda &A_{12}\lambda &\cdots &A_{1m}\lambda \\A_{21}\lambda &A_{22}\lambda &\cdots &A_{2m}\lambda \\\vdots &\vdots &\ddots &\vdots \\A_{n1}\lambda &A_{n2}\lambda &\cdots &A_{nm}\lambda \\\end{pmatrix}}\,.}
Piemērs ar reālu skaitli un matricu:
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{\displaystyle \lambda =2,\quad \mathbf {A} ={\begin{pmatrix}a&b\\c&d\\\end{pmatrix}}}
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{\displaystyle 2\mathbf {A} =2{\begin{pmatrix}a&b\\c&d\\\end{pmatrix}}={\begin{pmatrix}2\!\cdot \!a&2\!\cdot \!b\\2\!\cdot \!c&2\!\cdot \!d\\\end{pmatrix}}={\begin{pmatrix}a\!\cdot \!2&b\!\cdot \!2\\c\!\cdot \!2&d\!\cdot \!2\\\end{pmatrix}}={\begin{pmatrix}a&b\\c&d\\\end{pmatrix}}2=\mathbf {A} 2.}
Ja A n × m B m × p
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{\displaystyle \mathbf {A} ={\begin{pmatrix}A_{11}&A_{12}&\cdots &A_{1m}\\A_{21}&A_{22}&\cdots &A_{2m}\\\vdots &\vdots &\ddots &\vdots \\A_{n1}&A_{n2}&\cdots &A_{nm}\\\end{pmatrix}},\quad \mathbf {B} ={\begin{pmatrix}B_{11}&B_{12}&\cdots &B_{1p}\\B_{21}&B_{22}&\cdots &B_{2p}\\\vdots &\vdots &\ddots &\vdots \\B_{m1}&B_{m2}&\cdots &B_{mp}\\\end{pmatrix}}}
tad matricu reizinājums AB n × p [ 1] [ 2] [ 3] [ 4]
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{\displaystyle \mathbf {A} \mathbf {B} ={\begin{pmatrix}\left(\mathbf {AB} \right)_{11}&\left(\mathbf {AB} \right)_{12}&\cdots &\left(\mathbf {AB} \right)_{1p}\\\left(\mathbf {AB} \right)_{21}&\left(\mathbf {AB} \right)_{22}&\cdots &\left(\mathbf {AB} \right)_{2p}\\\vdots &\vdots &\ddots &\vdots \\\left(\mathbf {AB} \right)_{n1}&\left(\mathbf {AB} \right)_{n2}&\cdots &\left(\mathbf {AB} \right)_{np}\\\end{pmatrix}}}
kur katrs i, j Aik Bkj k = 1, 2, ..., m k
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{\displaystyle (\mathbf {A} \mathbf {B} )_{ij}=\sum _{k=1}^{m}A_{ik}B_{kj}\,.}
Tas nozīmē, ka reizinājums AB A B A B
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{\displaystyle {\overset {4\times 2{\text{ matrica}}}{\begin{bmatrix}{\color {Brown}{a_{11}}}&{\color {Brown}{a_{12}}}\\\cdot &\cdot \\{\color {Orange}{a_{31}}}&{\color {Orange}{a_{32}}}\\\cdot &\cdot \\\end{bmatrix}}}{\overset {2\times 3{\text{ matrica}}}{\begin{bmatrix}\cdot &{\color {Plum}{b_{12}}}&{\color {Violet}{b_{13}}}\\\cdot &{\color {Plum}{b_{22}}}&{\color {Violet}{b_{23}}}\\\end{bmatrix}}}={\overset {4\times 3{\text{ matrica}}}{\begin{bmatrix}\cdot &x_{12}&x_{13}\\\cdot &\cdot &\cdot \\\cdot &x_{32}&x_{33}\\\cdot &\cdot &\cdot \\\end{bmatrix}}}}
Vērtības krustpunktos ir iegūstamas ar šādām darbībām:
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{\displaystyle {\begin{aligned}x_{12}&={\color {Brown}{a_{11}}}{\color {Plum}{b_{12}}}+{\color {Brown}{a_{12}}}{\color {Plum}{b_{22}}}\\x_{13}&={\color {Brown}{a_{11}}}{\color {Violet}{b_{13}}}+{\color {Brown}{a_{12}}}{\color {Violet}{b_{23}}}\\x_{32}&={\color {Orange}{a_{31}}}{\color {Plum}{b_{12}}}+{\color {Orange}{a_{32}}}{\color {Plum}{b_{22}}}\\x_{33}&={\color {Orange}{a_{31}}}{\color {Violet}{b_{13}}}+{\color {Orange}{a_{32}}}{\color {Violet}{b_{23}}}\end{aligned}}}
Rindas matrica un kolonnas matrica Ja
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{\displaystyle \mathbf {A} ={\begin{pmatrix}a&b&c\end{pmatrix}}\,,\quad \mathbf {B} ={\begin{pmatrix}x\\y\\z\end{pmatrix}}\,,}
tad matricu reizināšanas rezultāts ir:
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{\displaystyle \mathbf {AB} ={\begin{pmatrix}a&b&c\end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}=ax+by+cz\,,}
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{\displaystyle \mathbf {BA} ={\begin{pmatrix}x\\y\\z\end{pmatrix}}{\begin{pmatrix}a&b&c\end{pmatrix}}={\begin{pmatrix}xa&xb&xc\\ya&yb&yc\\za&zb&zc\end{pmatrix}}\,.}
Jāievēro, ka AB BA 1 × 1 matrica, bet otrā — 3 × 3 matrica.
Kvadrātiska matrica un kolonnas matrica Ja
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{\displaystyle \mathbf {A} ={\begin{pmatrix}a&b&c\\p&q&r\\u&v&w\end{pmatrix}},\quad \mathbf {B} ={\begin{pmatrix}x\\y\\z\end{pmatrix}}\,,}
tad matricu reizināšanas rezultāts ir:
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{\displaystyle \mathbf {AB} ={\begin{pmatrix}a&b&c\\p&q&r\\u&v&w\end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}={\begin{pmatrix}ax+by+cz\\px+qy+rz\\ux+vy+wz\end{pmatrix}}\,,}
savukārt BA
Kvadrātiskas matricas Ja
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{\displaystyle \mathbf {A} ={\begin{pmatrix}a&b&c\\p&q&r\\u&v&w\end{pmatrix}},\quad \mathbf {B} ={\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}\,,}
tad matricu reizināšanas rezultāts ir:
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{\displaystyle \mathbf {AB} ={\begin{pmatrix}a&b&c\\p&q&r\\u&v&w\end{pmatrix}}{\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}={\begin{pmatrix}a\alpha +b\lambda +c\rho &a\beta +b\mu +c\sigma &a\gamma +b\nu +c\tau \\p\alpha +q\lambda +r\rho &p\beta +q\mu +r\sigma &p\gamma +q\nu +r\tau \\u\alpha +v\lambda +w\rho &u\beta +v\mu +w\sigma &u\gamma +v\nu +w\tau \end{pmatrix}}\,,}
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{\displaystyle \mathbf {BA} ={\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}{\begin{pmatrix}a&b&c\\p&q&r\\u&v&w\end{pmatrix}}={\begin{pmatrix}\alpha a+\beta p+\gamma u&\alpha b+\beta q+\gamma v&\alpha c+\beta r+\gamma w\\\lambda a+\mu p+\nu u&\lambda b+\mu q+\nu v&\lambda c+\mu r+\nu w\\\rho a+\sigma p+\tau u&\rho b+\sigma q+\tau v&\rho c+\sigma r+\tau w\end{pmatrix}}\,.}
Rindas matrica, kvadrātiska matrica un kolonnas matrica Ja
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{\displaystyle \mathbf {A} ={\begin{pmatrix}a&b&c\end{pmatrix}}\,,\quad \mathbf {B} ={\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}\,,\quad \mathbf {C} ={\begin{pmatrix}x\\y\\z\end{pmatrix}}\,,}
tad matricu reizināšanas rezultāts ir:
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{\displaystyle {\begin{aligned}\mathbf {ABC} &={\begin{pmatrix}a&b&c\end{pmatrix}}\left[{\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}\right]=\left[{\begin{pmatrix}a&b&c\end{pmatrix}}{\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}\right]{\begin{pmatrix}x\\y\\z\end{pmatrix}}\\&={\begin{pmatrix}a&b&c\end{pmatrix}}{\begin{pmatrix}\alpha x+\beta y+\gamma z\\\lambda x+\mu y+\nu z\\\rho x+\sigma y+\tau z\\\end{pmatrix}}={\begin{pmatrix}a\alpha +b\lambda +c\rho &a\beta +b\mu +c\sigma &a\gamma +b\nu +c\tau \end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}\\&=a\alpha x+b\lambda x+c\rho x+a\beta y+b\mu y+c\sigma y+a\gamma z+b\nu z+c\tau z\,,\end{aligned}}}
CBA A (BC ) = (AB )C
Taisnstūrveida matricas Ja
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{\displaystyle \mathbf {A} ={\begin{pmatrix}a&b&c\\x&y&z\end{pmatrix}}\,,\quad \mathbf {B} ={\begin{pmatrix}\alpha &\rho \\\beta &\sigma \\\gamma &\tau \\\end{pmatrix}}\,,}
tad matricu reizināšanas rezultāts ir:
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{\displaystyle \mathbf {A} \mathbf {B} ={\begin{pmatrix}a&b&c\\x&y&z\end{pmatrix}}{\begin{pmatrix}\alpha &\rho \\\beta &\sigma \\\gamma &\tau \\\end{pmatrix}}={\begin{pmatrix}a\alpha +b\beta +c\gamma &a\rho +b\sigma +c\tau \\x\alpha +y\beta +z\gamma &x\rho +y\sigma +z\tau \\\end{pmatrix}}\,,}
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{\displaystyle \mathbf {B} \mathbf {A} ={\begin{pmatrix}\alpha &\rho \\\beta &\sigma \\\gamma &\tau \\\end{pmatrix}}{\begin{pmatrix}a&b&c\\x&y&z\end{pmatrix}}={\begin{pmatrix}\alpha a+\rho x&\alpha b+\rho y&\alpha c+\rho z\\\beta a+\sigma x&\beta b+\sigma y&\beta c+\sigma z\\\gamma a+\tau x&\gamma b+\tau y&\gamma c+\tau z\end{pmatrix}}\,.}
Piemērs, kas parāda, ka matricu reizināšana nav komutatīva.
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{\displaystyle A={\bigl (}{\begin{smallmatrix}0&1\\0&0\end{smallmatrix}}{\bigr )},\,B={\bigl (}{\begin{smallmatrix}0&0\\0&1\end{smallmatrix}}{\bigr )},}
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{\displaystyle {\begin{aligned}A\cdot B&={\bigl (}{\begin{smallmatrix}0&1\\0&0\end{smallmatrix}}{\bigr )}\cdot {\bigl (}{\begin{smallmatrix}0&0\\0&1\end{smallmatrix}}{\bigr )}={\bigl (}{\begin{smallmatrix}0&1\\0&0\end{smallmatrix}}{\bigr )},\\B\cdot A&={\bigl (}{\begin{smallmatrix}0&0\\0&1\end{smallmatrix}}{\bigr )}\cdot {\bigl (}{\begin{smallmatrix}0&1\\0&0\end{smallmatrix}}{\bigr )}={\bigl (}{\begin{smallmatrix}0&0\\0&0\end{smallmatrix}}{\bigr )}.\end{aligned}}}
![{\displaystyle {\begin{aligned}\mathbf {ABC} &={\begin{pmatrix}a&b&c\end{pmatrix}}\left[{\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}\right]=\left[{\begin{pmatrix}a&b&c\end{pmatrix}}{\begin{pmatrix}\alpha &\beta &\gamma \\\lambda &\mu &\nu \\\rho &\sigma &\tau \\\end{pmatrix}}\right]{\begin{pmatrix}x\\y\\z\end{pmatrix}}\\&={\begin{pmatrix}a&b&c\end{pmatrix}}{\begin{pmatrix}\alpha x+\beta y+\gamma z\\\lambda x+\mu y+\nu z\\\rho x+\sigma y+\tau z\\\end{pmatrix}}={\begin{pmatrix}a\alpha +b\lambda +c\rho &a\beta +b\mu +c\sigma &a\gamma +b\nu +c\tau \end{pmatrix}}{\begin{pmatrix}x\\y\\z\end{pmatrix}}\\&=a\alpha x+b\lambda x+c\rho x+a\beta y+b\mu y+c\sigma y+a\gamma z+b\nu z+c\tau z\,,\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/43c6d62200e683fe070d5e66e7e94d11ffdfad2d)
