Multi-index notation


Multi-index notation is a mathematical notation that simplifies formulas used in multivariable calculus, partial differential equations and the theory of distributions, by generalising the concept of an integer index to an ordered tuple of indices.

Definition and basic propertiesEdit

An n-dimensional multi-index is an n-tuple


of non-negative integers (i.e. an element of the n-dimensional set of natural numbers, denoted  ).

For multi-indices   and   one defines:

Componentwise sum and difference
Partial order
Sum of components (absolute value)
Binomial coefficient
Multinomial coefficient
where  .
Higher-order partial derivative
where   (see also 4-gradient). Sometimes the notation   is also used.[1]

Some applicationsEdit

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  ).

Multinomial theorem
Multi-binomial theorem
Note that, since x + y is a vector and α is a multi-index, the expression on the left is short for (x1 + y1)α1⋯(xn + yn)αn.
Leibniz formula
For smooth functions f and g
Taylor series
For an analytic function f in n variables one has
In fact, for a smooth enough function, we have the similar Taylor expansion
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
General linear partial differential operator
A formal linear N-th order partial differential operator in n variables is written as
Integration by parts
For smooth functions with compact support in a bounded domain   one has
This formula is used for the definition of distributions and weak derivatives.

An example theoremEdit

If   are multi-indices and  , then



The proof follows from the power rule for the ordinary derivative; if α and β are in {0, 1, 2, …}, then







Suppose  ,  , and  . Then we have that


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

for each   and the theorem follows. Q.E.D.

See alsoEdit


  1. ^ Reed, M.; Simon, B. (1980). Methods of Modern Mathematical Physics: Functional Analysis I (Revised and enlarged ed.). San Diego: Academic Press. p. 319. ISBN 0-12-585050-6.
  • Saint Raymond, Xavier (1991). Elementary Introduction to the Theory of Pseudodifferential Operators. Chap 1.1 . CRC Press. ISBN 0-8493-7158-9

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