|
|
Line 1: |
Line 1: |
|
The [[mathematical notation]] of '''multi-indices''' simplifies formulae used in [[multivariable calculus]], [[partial differential equation]]s and the theory of [[distribution (mathematics)|distribution]]s, by generalising the concept of an integer [[index (mathematics)|index]]{{Disambiguation needed|date=February 2012}} to an ordered [[tuple]] of indices. |
|
The [[mathematical notation]] of '''multi-indices''' simplifies formulae used in [[multivariable calculus]], [[partial differential equation]]s and the theory of [[distribution (mathematics)|distribution]]s, by generalising the concept of non-negative integer [[index (mathematics)|index]] (in the sense of [[exponent]]) to an ordered [[tuple]] of indices. |
|
|
|
|
|
==Multi-index notation== |
|
==Multi-index notation== |
The mathematical notation of multi-indices simplifies formulae used in multivariable calculus, partial differential equations and the theory of distributions, by generalising the concept of non-negative integer index (in the sense of exponent) to an ordered tuple of indices.
Multi-index notation
An n-dimensional multi-index is an n-tuple
![{\displaystyle \alpha =(\alpha _{1},\alpha _{2},\ldots ,\alpha _{n})}](https://wikimedia.org/api/rest_v1/media/math/render/svg/70122d07b448b26cbd14d9542d648d5c761d3107)
of non-negative integers (i.e. an element of
). For multi-indices
and
one defines:
- Componentwise sum and difference
![{\displaystyle \alpha \pm \beta =(\alpha _{1}\pm \beta _{1},\,\alpha _{2}\pm \beta _{2},\ldots ,\,\alpha _{n}\pm \beta _{n})}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1d6b5d7d524ee5390e8ad81b8ee3d74d81261d70)
![{\displaystyle \alpha \leq \beta \quad \Leftrightarrow \quad \alpha _{i}\leq \beta _{i}\quad \forall \,i\in \{1,\ldots ,n\}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c5cbb846993d49f8992be68de9b846d277e187ec)
- Sum of components (absolute value)
![{\displaystyle |\alpha |=\alpha _{1}+\alpha _{2}+\cdots +\alpha _{n}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7832f93f42cce41671070ecf5a4135255fb4c93a)
![{\displaystyle \alpha !=\alpha _{1}!\cdot \alpha _{2}!\cdots \alpha _{n}!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2c5ec408016ade71f03fa953438c6e9560a32a05)
![{\displaystyle {\binom {\alpha }{\beta }}={\binom {\alpha _{1}}{\beta _{1}}}{\binom {\alpha _{2}}{\beta _{2}}}\cdots {\binom {\alpha _{n}}{\beta _{n}}}={\frac {\alpha !}{\beta !(\alpha -\beta )!}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b7812d634fd363aaf49931bf182ee1b6b1b4657f)
where ![{\displaystyle |\alpha |=k\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0b5b5d5bee473c980727adc72f1e8fa5f5942b1a)
![{\displaystyle x^{\alpha }=x_{1}^{\alpha _{1}}x_{2}^{\alpha _{2}}\ldots x_{n}^{\alpha _{n}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5fdd8717184ad0646712c80da26c6abc93862e14)
where ![{\displaystyle \partial _{i}^{\alpha _{i}}:=\partial ^{\alpha _{i}}/\partial x_{i}^{\alpha _{i}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e28aeb23d73561e2073c54f9fef980f15745b1c8)
Some applications
The multi-index notation allows the extension of many formulae from elementary calculus to the corresponding multi-variable case. Below are some examples. In all the following,
(or
),
, and
(or
).
![{\displaystyle {\biggl (}\sum _{i=1}^{n}x_{i}{\biggr )}^{k}=\sum _{|\alpha |=k}{\binom {k}{\alpha }}\,x^{\alpha }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1c144ffc0893748b9c4119fe1ed81d4bb914846f)
![{\displaystyle (x+y)^{\alpha }=\sum _{\nu \leq \alpha }{\binom {\alpha }{\nu }}\,x^{\nu }y^{\alpha -\nu }.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/db39d7304e8fba5771d4a36ee307044d1b1df7b4)
For smooth functions f and g
![{\displaystyle \partial ^{\alpha }(fg)=\sum _{\nu \leq \alpha }{\binom {\alpha }{\nu }}\,\partial ^{\nu }f\,\partial ^{\alpha -\nu }g.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/9ef0670fad1007b937c3a9a82a6c497046feb1d6)
For an analytic function f in n variables one has
![{\displaystyle f(x+h)=\sum _{\alpha \in \mathbb {N} _{0}^{n}}^{}{{\frac {\partial ^{\alpha }f(x)}{\alpha !}}h^{\alpha }}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/876d4e484fff118421fa290f089b77c8e1bdda3e)
In fact, for a smooth enough function, we have the similar Taylor expansion
![{\displaystyle f(x+h)=\sum _{|\alpha |\leq n}{{\frac {\partial ^{\alpha }f(x)}{\alpha !}}h^{\alpha }}+R_{n}(x,h),}](https://wikimedia.org/api/rest_v1/media/math/render/svg/9527adc0a8f66e4a1dde784952c7185e1718e11c)
where the last term (the remainder) depends on the exact version of Taylor's formula. For instance, for the Cauchy formula (with integral remainder), one gets
![{\displaystyle R_{n}(x,h)=(n+1)\sum _{|\alpha |=n+1}{\frac {h^{\alpha }}{\alpha !}}\int _{0}^{1}(1-t)^{n}\partial ^{\alpha }f(x+th)\,dt.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e7c54a9fbc554936f0927b1cf90b89c86aa506b0)
A formal N-th order partial differential operator in n variables is written as
![{\displaystyle P(\partial )=\sum _{|\alpha |\leq N}{}{a_{\alpha }(x)\partial ^{\alpha }}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8337065ee25c08f25c133f4ddb728bd9e8dd8231)
For smooth functions with compact support in a bounded domain
one has
![{\displaystyle \int _{\Omega }{}{u(\partial ^{\alpha }v)}\,dx=(-1)^{|\alpha |}\int _{\Omega }^{}{(\partial ^{\alpha }u)v\,dx}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/6542070b9eaa44eaab4d070c9706900d637faf26)
This formula is used for the definition of distributions and weak derivatives.
An example theorem
If
are multi-indices and
, then
![{\displaystyle \partial ^{\alpha }x^{\beta }={\begin{cases}{\frac {\beta !}{(\beta -\alpha )!}}x^{\beta -\alpha }&{\hbox{if}}\,\,\alpha \leq \beta ,\\0&{\hbox{otherwise.}}\end{cases}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/03bfecd43d58ed90056fcb96123644095c3db8f1)
Proof
The proof follows from the power rule for the ordinary derivative; if α and β are in {0, 1, 2, . . .}, then
![{\displaystyle {\frac {d^{\alpha }}{dx^{\alpha }}}x^{\beta }={\begin{cases}{\frac {\beta !}{(\beta -\alpha )!}}x^{\beta -\alpha }&{\hbox{if}}\,\,\alpha \leq \beta ,\\0&{\hbox{otherwise.}}\end{cases}}\qquad (1)}](https://wikimedia.org/api/rest_v1/media/math/render/svg/31e5c6a7714c8b0ef8a987d47fb5d7620faa480c)
Suppose
,
, and
. Then we have that
![{\displaystyle {\begin{aligned}\partial ^{\alpha }x^{\beta }&={\frac {\partial ^{\vert \alpha \vert }}{\partial x_{1}^{\alpha _{1}}\cdots \partial x_{n}^{\alpha _{n}}}}x_{1}^{\beta _{1}}\cdots x_{n}^{\beta _{n}}\\&={\frac {\partial ^{\alpha _{1}}}{\partial x_{1}^{\alpha _{1}}}}x_{1}^{\beta _{1}}\cdots {\frac {\partial ^{\alpha _{n}}}{\partial x_{n}^{\alpha _{n}}}}x_{n}^{\beta _{n}}.\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3c5a4a78a610adf197ab62f8f42c3a9924a92b2e)
For each i in {1, . . ., n}, the function
only depends on
. In the above, each partial differentiation
therefore reduces to the corresponding ordinary differentiation
. Hence, from equation (1), it follows that
vanishes if αi > βi for at least one i in {1, . . ., n}. If this is not the case, i.e., if α ≤ β as multi-indices, then
![{\displaystyle {\frac {d^{\alpha _{i}}}{dx_{i}^{\alpha _{i}}}}x_{i}^{\beta _{i}}={\frac {\beta _{i}!}{(\beta _{i}-\alpha _{i})!}}x_{i}^{\beta _{i}-\alpha _{i}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/71dc5027a5bbd718f053ca6514950f6e600c6087)
for each
and the theorem follows.
References
- Saint Raymond, Xavier (1991). Elementary Introduction to the Theory of Pseudodifferential Operators. Chap 1.1 . CRC Press. ISBN 0-8493-7158-9
multi-index derivative of a power at PlanetMath.