Faà di Bruno's formula


Faà di Bruno's formula is an identity in mathematics generalizing the chain rule to higher derivatives. It is named after, although he was not the first to state or prove the formula. In 1800, more than 50 years before Faà di Bruno, the French mathematician Louis François Antoine Arbogast had stated the formula in a calculus textbook, which is considered to be the first published reference on the subject.
Perhaps the most well-known form of Faà di Bruno's formula says that
where the sum is over all -tuples of nonnegative integers satisfying the constraint
Sometimes, to give it a memorable pattern, it is written in a way in which the coefficients that have the combinatorial interpretation discussed below are less explicit:
Combining the terms with the same value of
and noticing that has to be zero for leads to a somewhat simpler formula expressed in terms of partial exponential Bell polynomials
This formula works for all, however for the polynomials are zero and thus summation in the formula can start with.

Combinatorial form

The formula has a "combinatorial" form:
where
  • runs through the set of all partitions of the set,
  • "" means the variable runs through the list of all of the "blocks" of the partition, and
  • denotes the cardinality of the set .

    Example

The following is a concrete explanation of the combinatorial form for the case.
The pattern is:
The factor corresponds to the partition 2 + 1 + 1 of the integer 4, in the obvious way. The factor that goes with it corresponds to the fact that there are three summands in that partition. The coefficient 6 that goes with those factors corresponds to the fact that there are exactly six partitions of a set of four members that break it into one part of size 2 and two parts of size 1.
Similarly, the factor in the third line corresponds to the partition 2 + 2 of the integer 4,, while corresponds to the fact that there are two summands in that partition. The coefficient 3 corresponds to the fact that there are ways of partitioning 4 objects into groups of 2. The same concept applies to the others.
A memorizable scheme is as follows:

Variations

Multivariate version

Let. Then the following identity holds regardless of whether the variables are all distinct, or all identical, or partitioned into several distinguishable classes of indistinguishable variables :
where
  • runs through the set of all partitions of the set,
  • "" means the variable runs through the list of all of the "blocks" of the partition, and
  • denotes the cardinality of the set .
More general versions hold for cases where the all functions are vector- and even Banach-space-valued. In this case one needs to consider the Fréchet derivative or Gateaux derivative.
; Example
The five terms in the following expression correspond in the obvious way to the five partitions of the set, and in each case the order of the derivative of is the number of parts in the partition:
If the three variables are indistinguishable from each other, then three of the five terms above are also indistinguishable from each other, and then we have the classic one-variable formula.

Formal power series version

Suppose and are formal power series and.
Then the composition is again a formal power series,
where and the other coefficient for
can be expressed as a sum over compositions of or as an equivalent sum over integer partitions of :
where
is the set of compositions of with denoting the number of parts,
or
where
is the set of partitions of into parts, in frequency-of-parts form.
The first form is obtained by picking out the coefficient of
in "by inspection", and the second form
is then obtained by collecting like terms, or alternatively, by applying the multinomial theorem.
The special case, gives the exponential formula.
The special case,
gives an expression for the reciprocal of the formal power series
in the case.
Stanley
gives a version for exponential power series.
In the formal power series
we have the th derivative at 0:
This should not be construed as the value of a function, since these series are purely formal; there is no such thing as convergence or divergence in this context.
If
and
and
then the coefficient is given by
where runs through the set of all partitions of the set and
are the blocks of the partition, and
is the number of members of the th block, for.
This version of the formula is particularly well suited to the purposes of combinatorics.
We can also write with respect to the notation above
where are Bell polynomials.

A special case

If, then all of the derivatives of are the same and are a factor common to every term:
where is the nth complete exponential Bell polynomial.
In case is a cumulant-generating function, then
is a moment-generating function, and the polynomial in various derivatives of is the polynomial that expresses the moments as functions of the cumulants.

Historical surveys and essays

  • . "The mathematical work" is an essay on the mathematical activity, describing both the research and teaching activity of Francesco Faà di Bruno.
  • .
  • .

    Research works

  • , Entirely freely available from Google books.
  • . Entirely freely available from Google books. A well-known paper where Francesco Faà di Bruno presents the two versions of the formula that now bears his name, published in the journal founded by Barnaba Tortolini.
  • . Entirely freely available from Google books.
  • . Entirely freely available from Google books.
  • Flanders, Harley "From Ford to Faa", American Mathematical Monthly 108: 558–61
  • .
  • .
  • , available at . This paper, according to is one of the precursors of : note that the author signs only as "T.A.", and the attribution to J. F. C. Tiburce Abadie is due again to Johnson.
  • , available at . This paper, according to is one of the precursors of : note that the author signs only as "A.", and the attribution to J. F. C. Tiburce Abadie is due again to Johnson.