General Leibniz rule

For other uses, see Leibniz's rule (disambiguation).

In calculus, the general Leibniz rule, named after Gottfried Wilhelm Leibniz, generalizes the product rule (which is also known as "Leibniz's rule"). It states that if $f$ and $g$ are $n$-times differentiable functions, then the product $fg$ is also $n$-times differentiable and its $n$th derivative is given by

$(fg)^{(n)}=\sum _{k=0}^{n}{n \choose k}f^{(n-k)}g^{(k)},$

where ${n \choose k}={n! \over k!(n-k)!}$ is the binomial coefficient and $f^{(j)}$ denotes the jth derivative of f (and in particular $f^{(0)}=f$).

The rule can be proven by using the product rule and mathematical induction.

Second derivative

If, for example, n = 2, the rule gives an expression for the second derivative of a product of two functions:

$(fg)''(x)=\sum \limits _{k=0}^{2}{{\binom {2}{k}}f^{(2-k)}(x)g^{(k)}(x)}=f''(x)g(x)+2f'(x)g'(x)+f(x)g''(x).$

More than two factors

The formula can be generalized to the product of m differentiable functions f₁,...,f_m.

$\left(f_{1}f_{2}\cdots f_{m}\right)^{(n)}=\sum _{k_{1}+k_{2}+\cdots +k_{m}=n}{n \choose k_{1},k_{2},\ldots ,k_{m}}\prod _{1\leq t\leq m}f_{t}^{(k_{t})}\,,$

where the sum extends over all m-tuples (k₁,...,k_m) of non-negative integers with $\sum _{t=1}^{m}k_{t}=n,$ and

${n \choose k_{1},k_{2},\ldots ,k_{m}}={\frac {n!}{k_{1}!\,k_{2}!\cdots k_{m}!}}$

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 $f$ and $g$ be $n$-times differentiable functions. The base case when $n=1$ claims that:

$(fg)'=f'g+fg',$

which is the usual product rule and is known to be true. Next, assume that the statement holds for a fixed $n\geq 1,$ that is, that

$(fg)^{(n)}=\sum _{k=0}^{n}{\binom {n}{k}}f^{(n-k)}g^{(k)}.$

Then,

$n+1,$