General Leibniz rule
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In calculus, the general Leibniz rule,[1] named after Gottfried Wilhelm Leibniz, generalizes the product rule (which is also known as "Leibniz's rule"). It states that if and are n-times differentiable functions, then the product is also n-times differentiable and its n-th derivative is given by where is the binomial coefficient and denotes the jth derivative of f (and in particular ).
The rule can be proven by using the product rule and mathematical induction.
Second derivative
[edit]If, for example, n = 2, the rule gives an expression for the second derivative of a product of two functions:
More than two factors
[edit]The formula can be generalized to the product of m differentiable functions f1,...,fm. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle \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\le t\le m}f_{t}^{(k_{t})}\,,} where the sum extends over all m-tuples (k1,...,km) of non-negative integers with and are the multinomial coefficients. This is akin to the multinomial formula from algebra.
Proof
[edit]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
[edit]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 (with coefficients that are differentiable sufficiently many times) 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.
See also
[edit]- Binomial theorem – Algebraic expansion of powers of a binomial
- Derivation (differential algebra) – Algebraic generalization of the derivative
- Derivative – Instantaneous rate of change (mathematics)
- Differential algebra – Algebraic study of differential equations
- Pascal's triangle – Triangular array of the binomial coefficients in mathematics
- Product rule – Formula for the derivative of a product
- Quotient rule – Formula for the derivative of a ratio of functions
References
[edit]- ^ Olver, Peter J. (2000). Applications of Lie Groups to Differential Equations. Springer. pp. 318–319. ISBN 9780387950006.