General Leibniz rule


In calculus, the general Leibniz rule, named after Gottfried Wilhelm Leibniz, generalizes the product rule. It states that if and are -times differentiable functions, then the product is also -times differentiable and its th derivative is given by
where is the binomial coefficient and denotes the jth derivative of f.
The rule can be proved by using the product rule and mathematical induction.

Second derivative

If, for example,, the rule gives an expression for the second derivative of a product of two functions:

More than two factors

The formula can be generalized to the product of m differentiable functions f1,...,fm.
where the sum extends over all m-tuples of non-negative integers with and
are the multinomial coefficients. This is akin to the multinomial formula from algebra.

Proof

The proof of the general Leibniz rule proceeds by induction. Let and be -times differentiable functions. The base case when claims that:
which is the usual product rule and is known to be true. Next, assume that the statement holds for a fixed that is, that
Then,
And so the statement holds for and the proof is complete.

Multivariable calculus

With the multi-index notation for partial derivatives of functions of several variables, the Leibniz rule states more generally:
This formula can be used to derive a formula that computes the symbol of the composition of differential operators. In fact, let P and Q be differential operators and Since R is also a differential operator, the symbol of R is given by:
A direct computation now gives:
This formula is usually known as the Leibniz formula. It is used to define the composition in the space of symbols, thereby inducing the ring structure.