Multiindex

In der Mathematik fasst man häufig mehrere Indizes zu einem einzigen Multiindex zusammen. Formal gesehen ist ein Multiindex ${\displaystyle {\boldsymbol {\alpha }}=(\alpha _{1},\ldots ,\alpha _{n})}$ ein Tupel natürlicher Zahlen.

Verallgemeinert m​an Formeln v​on einer Variable a​uf mehrere Variablen, s​o ist e​s aus notationstechnischen Gründen m​eist sinnvoll, d​ie Multiindexschreibweise z​u verwenden. Ein Beispiel wäre e​ine Potenzreihe m​it einer Veränderlichen a​uf Mehrfachpotenzreihen umzuschreiben. Multiindizes werden häufig i​n der mehrdimensionalen Analysis u​nd Theorie d​er Distributionen verwendet.

Konventionen der Multiindex-Schreibweise

In diesem Abschnitt seien ${\displaystyle {\boldsymbol {\alpha }}=(\alpha _{1},\ldots ,\alpha _{n}),\ {\boldsymbol {k}}=(k_{1},\ldots ,k_{n}),\ {\boldsymbol {\ell }}=(\ell _{1},\ldots ,\ell _{n})\in \mathbb {N} _{0}^{n}}$ jeweils ${\displaystyle n}$-Tupel natürlicher Zahlen. Für die Multiindex-Schreibweise werden üblicherweise die folgenden Konventionen vereinbart:

${\displaystyle {\begin{array}{ccl}{\boldsymbol {k}}={\boldsymbol {\ell }}&\iff &k_{1}=\ell _{1}\;,\;\ldots \;,\;k_{n}=\ell _{n}\\\\{\boldsymbol {k}}\leq {\boldsymbol {\ell }}&\iff &k_{1}\leq \ell _{1}\;,\;\ldots \;,\;k_{n}\leq \ell _{n}\\\\{\boldsymbol {k}}+{\boldsymbol {\ell }}&:=&(k_{1}+\ell _{1}\;,\;\ldots \;,\;k_{n}+\ell _{n})\\\\{\boldsymbol {k}}!&:=&k_{1}!\cdots k_{n}!\\\\{{\boldsymbol {\alpha }} \choose {\boldsymbol {k}}}&:=&{\frac {{\boldsymbol {\alpha }}!}{({\boldsymbol {\alpha -k}})!\,{\boldsymbol {k}}!}}={\alpha _{1} \choose k_{1}}\cdots {\alpha _{n} \choose k_{n}}\\\\|{\boldsymbol {k}}|&:=&k_{1}+\cdots +k_{n}\\\\{\boldsymbol {x}}^{\boldsymbol {k}}&:=&x_{1}^{k_{1}}\cdots x_{n}^{k_{n}}\\\\{\boldsymbol {D}}^{\boldsymbol {k}}&:=&D_{1}^{k_{1}}\cdots D_{n}^{k_{n}}\,,\end{array}}}$

wobei ${\displaystyle {\boldsymbol {x}}\in \mathbb {C} ^{n}}$ und ${\displaystyle {\boldsymbol {D}}}$ einen Differentialoperator bezeichnet.

Anwendungsbeispiele

Potenzreihe

Eine Mehrfachpotenzreihe ${\displaystyle \sum _{k_{1}\geq 0}\cdots \sum _{k_{n}\geq 0}a_{k_{1},\ldots ,k_{n}}(z_{1}-z_{1}^{o})^{k_{1}}\cdots (z_{n}-z_{n}^{o})^{k_{n}}}$ lässt sich kurz schreiben als ${\displaystyle \sum _{{\boldsymbol {k}}\geq 0}a_{\boldsymbol {k}}({\boldsymbol {z}}-{\boldsymbol {z}}^{o})^{\boldsymbol {k}}}$.

Potenzfunktion

Ist ${\displaystyle {\boldsymbol {x}}\in \mathbb {R} ^{n}}$ und sind ${\displaystyle {\boldsymbol {k}},{\boldsymbol {m}}\in \mathbb {N} ^{n}}$, so gilt ${\displaystyle {\boldsymbol {D}}^{\boldsymbol {k}}{\frac {{\boldsymbol {x}}^{\boldsymbol {m}}}{{\boldsymbol {m}}!}}={\frac {{\boldsymbol {x}}^{{\boldsymbol {m}}-{\boldsymbol {k}}}}{({\boldsymbol {m}}-{\boldsymbol {k}})!}}}$ und ${\displaystyle {\boldsymbol {D}}^{\boldsymbol {k}}{\frac {|{\boldsymbol {x}}|^{m}}{m!}}={\frac {|{\boldsymbol {x}}|^{m-|{\boldsymbol {k}}|}}{(m-|{\boldsymbol {k}}|)!}}}$.

Geometrische Reihe

Für ${\displaystyle -{\boldsymbol {1}}<{\boldsymbol {x}}<{\boldsymbol {1}}}$ gilt ${\displaystyle \sum _{|{\boldsymbol {k}}|\geq 0}{\boldsymbol {x}}^{\boldsymbol {k}}={\frac {1}{({\boldsymbol {1}}-{\boldsymbol {x}})^{\boldsymbol {1}}}}}$, wobei ${\displaystyle {\boldsymbol {1}}=(1,\ldots ,1)}$ ist.

Binomischer Lehrsatz

Sind ${\displaystyle {\boldsymbol {x}},{\boldsymbol {y}}\in \mathbb {C} ^{n}}$ und ist ${\displaystyle {\boldsymbol {m}}\in \mathbb {N} ^{n}}$, so gilt ${\displaystyle ({\boldsymbol {x}}+{\boldsymbol {y}})^{\boldsymbol {m}}=\sum _{{\boldsymbol {k}}\leq {\boldsymbol {m}}}{{\boldsymbol {m}} \choose {\boldsymbol {k}}}{\boldsymbol {x}}^{\boldsymbol {k}}{\boldsymbol {y}}^{{\boldsymbol {m}}-{\boldsymbol {k}}}}$ bzw. ${\displaystyle {\frac {({\boldsymbol {x}}+{\boldsymbol {y}})^{\boldsymbol {m}}}{{\boldsymbol {m}}!}}=\sum _{{\boldsymbol {k}}+{\boldsymbol {j}}={\boldsymbol {m}}}{\frac {{\boldsymbol {x}}^{\boldsymbol {k}}}{{\boldsymbol {k}}!}}{\frac {{\boldsymbol {y}}^{\boldsymbol {j}}}{{\boldsymbol {j}}!}}}$.

Multinomialtheorem

Für ${\displaystyle {\boldsymbol {x}}=(x_{1},\ldots ,x_{n})\in \mathbb {R} ^{n}}$ und ${\displaystyle m\in \mathbb {N} }$ ist ${\displaystyle (x_{1}+\cdots +x_{n})^{m}=\sum _{k_{1}+\cdots +k_{n}=m}{m \choose k_{1},\ldots ,k_{n}}x_{1}^{k_{1}}\cdots x_{n}^{k_{n}}}$ bzw. ${\displaystyle {\frac {(x_{1}+\cdots +x_{n})^{m}}{m!}}=\sum _{k_{1}+\cdots +k_{n}=m}{\frac {x_{1}^{k_{1}}}{k_{1}!}}\cdots {\frac {x_{n}^{k_{n}}}{k_{n}!}}}$, was sich kurz schreiben lässt als ${\displaystyle {\frac {|{\boldsymbol {x}}|^{m}}{m!}}=\sum _{|{\boldsymbol {k}}|=m}{\frac {{\boldsymbol {x}}^{\boldsymbol {k}}}{{\boldsymbol {k}}!}}}$.

Leibniz-Regel

Ist ${\displaystyle {\boldsymbol {m}}\in \mathbb {N} ^{n}}$ und sind ${\displaystyle f,g\colon \mathbb {R} ^{n}\to \mathbb {R} }$ m-mal stetig differenzierbare Funktionen, so gilt

${\displaystyle (fg)^{({\boldsymbol {m}})}=\sum _{{\boldsymbol {k}}\leq {\boldsymbol {m}}}{{\boldsymbol {m}} \choose {\boldsymbol {k}}}f^{({\boldsymbol {k}})}g^{({\boldsymbol {m}}-{\boldsymbol {k}})}}$

beziehungsweise

${\displaystyle {\frac {(fg)^{({\boldsymbol {m}})}}{{\boldsymbol {m}}!}}=\sum _{{\boldsymbol {k}}+{\boldsymbol {j}}={\boldsymbol {m}}}{\frac {f^{({\boldsymbol {k}})}}{{\boldsymbol {k}}!}}{\frac {g^{({\boldsymbol {j}})}}{{\boldsymbol {j}}!}}}$.

Diese Identität heißt Leibniz-Regel.

Und sind ${\displaystyle f_{1},\ldots ,f_{n}\colon \mathbb {R} \to \mathbb {R} }$ m-mal stetig differenzierbare Funktionen, so ist

${\displaystyle {\frac {(f_{1}\cdots f_{n})^{m}}{m!}}=\sum _{|{\boldsymbol {k}}|=m}{\frac {{\boldsymbol {f}}^{({\boldsymbol {k}})}}{{\boldsymbol {k}}!}}}$,

wobei ${\displaystyle {\boldsymbol {f}}^{({\boldsymbol {k}})}=(f_{1},\ldots ,f_{n})^{{\big (}(k_{1}),\ldots ,(k_{n}){\big )}}=f_{1}^{(k_{1})}\cdots f_{n}^{(k_{n})}}$ ist.

Cauchy-Produkt

Für Mehrfachpotenzreihen ${\displaystyle f({\boldsymbol {z}})=\sum _{|{\boldsymbol {\ell }}|\geq 0}a_{\boldsymbol {\ell }}\,{\boldsymbol {z}}^{\boldsymbol {\ell }}\;,\;g({\boldsymbol {z}})=\sum _{|{\boldsymbol {\ell }}|\geq 0}b_{\boldsymbol {\ell }}\,{\boldsymbol {z}}^{\boldsymbol {\ell }}}$ gilt ${\displaystyle f({\boldsymbol {z}})\,g({\boldsymbol {z}})=\sum _{|{\boldsymbol {\ell }}|\geq 0}\left(\sum _{{\boldsymbol {k}}+{\boldsymbol {j}}={\boldsymbol {\ell }}}a_{\boldsymbol {k}}\,b_{\boldsymbol {j}}\right){\boldsymbol {z}}^{\boldsymbol {\ell }}}$.

Sind ${\displaystyle f_{1}(z)=\sum _{\ell =0}^{\infty }a_{1\ell }z^{\ell }\;,\;\ldots \;,\;f_{n}(z)=\sum _{\ell =0}^{\infty }a_{n\ell }z^{\ell }}$ Potenzreihen einer Veränderlichen, so gilt ${\displaystyle f_{1}(z)\cdots f_{n}(z)=\sum _{\ell =0}^{\infty }\left(\sum _{|{\boldsymbol {k}}|=\ell }a_{\boldsymbol {k}}\right)z^{\ell }}$, wobei ${\displaystyle a_{\boldsymbol {k}}=a_{1k_{1}}\cdots a_{nk_{n}}}$ ist.

Exponentialreihe

Für ${\displaystyle {\boldsymbol {z}}=(z_{1},...,z_{n})\in \mathbb {C} ^{n}}$ gilt ${\displaystyle e^{z_{1}+...+z_{n}}=\sum _{{\boldsymbol {k}}\geq 0}{\frac {{\boldsymbol {z}}^{\boldsymbol {k}}}{{\boldsymbol {k}}!}}}$.

Binomische Reihe

Sind ${\displaystyle {\boldsymbol {\alpha }},{\boldsymbol {x}}\in \mathbb {C} ^{n}}$ und sind alle Komponenten von ${\displaystyle {\boldsymbol {x}}}$ betragsmäßig ${\displaystyle <1\,}$, so gilt ${\displaystyle ({\boldsymbol {1}}+{\boldsymbol {x}})^{\boldsymbol {\alpha }}=\sum _{|{\boldsymbol {k}}|\geq 0}{{\boldsymbol {\alpha }} \choose {\boldsymbol {k}}}\,{\boldsymbol {x}}^{\boldsymbol {k}}}$.

Vandermondesche Konvolution

Ist ${\displaystyle {\boldsymbol {m}}\in \mathbb {N} ^{n}}$ und sind ${\displaystyle {\boldsymbol {\alpha }},{\boldsymbol {\beta }}\in \mathbb {C} ^{n}}$, so gilt ${\displaystyle {{\boldsymbol {\alpha }}+{\boldsymbol {\beta }} \choose {\boldsymbol {m}}}=\sum _{{\boldsymbol {k}}\leq {\boldsymbol {m}}}{{\boldsymbol {\alpha }} \choose {\boldsymbol {k}}}{{\boldsymbol {\beta }} \choose {\boldsymbol {m}}-{\boldsymbol {k}}}=\sum _{{\boldsymbol {k}}+{\boldsymbol {j}}={\boldsymbol {m}}}{{\boldsymbol {\alpha }} \choose {\boldsymbol {k}}}{{\boldsymbol {\beta }} \choose {\boldsymbol {j}}}}$.

Ist ${\displaystyle m\in \mathbb {N} }$ und ${\displaystyle {\boldsymbol {\alpha }}=(\alpha _{1},...,\alpha _{n})\in \mathbb {C} ^{n}}$, so gilt ${\displaystyle {|{\boldsymbol {\alpha }}| \choose m}=\sum _{|{\boldsymbol {k}}|=m}{{\boldsymbol {\alpha }} \choose {\boldsymbol {k}}}}$.

Cauchysche Integralformel

In mehreren Veränderlichen ${\displaystyle z_{1},\ldots ,z_{n}\,}$ lässt sich die cauchysche Integralformel

${\displaystyle {\frac {D^{\boldsymbol {k}}f(z_{1},\ldots ,z_{n})}{{\boldsymbol {k}}!}}={\frac {1}{(2\pi i)^{n}}}\oint _{\partial U_{n}}\cdots \oint _{\partial U_{1}}{\frac {f(\xi _{1},\ldots ,\xi _{n})}{(\xi _{1}-z_{1})^{k_{1}+1}\cdots (\xi _{n}-z_{n})^{k_{n}+1}}}d\xi _{1}\cdots d\xi _{n}}$

kurz schreiben a​ls

${\displaystyle a_{\boldsymbol {k}}:={\frac {D^{\boldsymbol {k}}f({\boldsymbol {z}})}{{\boldsymbol {k}}!}}={\frac {1}{(2\pi i)^{\boldsymbol {1}}}}\oint _{\partial {\boldsymbol {U}}}{\frac {f({\boldsymbol {\xi }})}{({\boldsymbol {\xi }}-{\boldsymbol {z}})^{{\boldsymbol {k}}+{\boldsymbol {1}}}}}\,{\boldsymbol {d\xi }}}$,

wobei ${\displaystyle \partial {\boldsymbol {U}}=\partial U_{1}\times \cdots \times \partial U_{n}}$ sein soll. Ebenso gilt die Abschätzung ${\displaystyle |a_{\boldsymbol {k}}|\leq {\tfrac {M}{{\boldsymbol {r}}^{\boldsymbol {k}}}}}$, wobei ${\displaystyle \textstyle M=\max _{{\boldsymbol {\xi }}\in \partial {\boldsymbol {U}}}|f({\boldsymbol {k}})|}$ ist.

Taylor-Reihe

Ist ${\displaystyle f\colon \mathbb {R} ^{n}\to \mathbb {R} }$ eine analytische Funktion oder ${\displaystyle f\colon \mathbb {C} ^{n}\to \mathbb {C} }$ eine holomorphe Abbildung, so kann man diese Funktion in eine Taylor-Reihe

${\displaystyle f({\boldsymbol {z}})=\sum _{|{\boldsymbol {k}}|\geq 0}{\frac {D^{\boldsymbol {k}}f({\boldsymbol {z}}^{o})}{{\boldsymbol {k}}!}}(z-z^{o})^{\boldsymbol {k}}}$

entwickeln, wobei ${\displaystyle {\boldsymbol {k}}}$ ein Multiindex ist.

Hurwitz-Identität

Für ${\displaystyle x,y\in \mathbb {C} }$ mit ${\displaystyle x\neq 0}$ und ${\displaystyle {\boldsymbol {a}}=(a_{1},...,a_{n})\in \mathbb {C} ^{n}}$ gilt ${\displaystyle (x+y)^{n}=\sum _{{\boldsymbol {0}}\leq {\boldsymbol {k}}\leq {\boldsymbol {1}}}x\,(x+{\boldsymbol {a}}\cdot {\boldsymbol {k}})^{|{\boldsymbol {k}}|-1}\,(y-{\boldsymbol {a}}\cdot {\boldsymbol {k}})^{n-|{\boldsymbol {k}}|}}$.

Dies verallgemeinert die Abelsche Identität ${\displaystyle (x+y)^{n}=\sum _{k=0}^{n}{n \choose k}\,x\,(x+ak)^{k-1}\,(y-ak)^{n-k}}$.

Letztere erhält man im Fall ${\displaystyle {\boldsymbol {a}}=(a,a,...,a)}$.

Literatur

• Otto Forster: Analysis. Band 2: Differentialrechnung im Rⁿ. Gewöhnliche Differentialgleichungen. 7. verbesserte Auflage. Vieweg + Teubner, Wiesbaden 2006, ISBN 3-8348-0250-6 (Vieweg Studium. Grundkurs Mathematik).
• Konrad Königsberger: Analysis. Band 2. 3. überarbeitete Auflage. Springer-Verlag, Berlin u. a. 2000, ISBN 3-540-66902-7.