Let be a formal power series. There exists coefficients such thatThe Jabotinsky matrix of is defined as the infinite matrix
When , becomes an infinite lower triangular matrix whose entries are given by ordinary Bell polynomials evaluated at the coefficients of . This is why is sometimes referred to as a Bell matrix.
History
Jabotinsky matrices have a long history, and were perhaps used for the first time in the context of iteration theory by Albert A. Bennett in 1915. Jabotinsky later pursued Bennett's research and applied them to Faber polynomials. Jabotinsky matrices were popularized during the 70s by Louis Comtet[fr]'s book Advanced Combinatorics, where he referred to them as iteration matrices (which is a denomination also sometimes used nowadays). This article's denomination appeared later. Donald Knuth uses the name convolution matrix.
Properties
Jabotinsky matrices satisfy the fundamental relationshipwhich makes the Jabotinsky matrix a (direct) representation of . Here the term denotes the composition of functions .
Given a sequence , we can instead define the matrix with the coefficient byIf is the constant sequence equal to , we recover Jabotinsky matrices. In some contexts, the sequence is chosen to be , so that the entry are given by regular Bell polynomials. This is a more convenient form for functions such as and where Stirling numbers of the first and second kind appear in the matrices (see the examples).
This generalization gives a completely equivalent matrix since .
Some authors, which include Bennett and Jabotinsky in their original work, use the transpose convention for the matrix, which is an infinite upper triangular matrix when . The fundamental relationship becomeswhich preserves the order of the operations.
Related matrices
Jabotinsky matrices are a special case of the later introduced Riordan arrays.
↑Aldrovandi, R. (2001). Special matrices of mathematical physics: stochastic, circulant, and Bell matrices. Singapore; River Edge, N.J: World Scientific. ISBN978-981-02-4708-9.
↑Jabotinsky, Eri (1947). "Sur la représentation de la composition de fonctions par un produit de matrices. Applicaton à l'itération de e^x et de e^x-1". Comptes rendus de l'Académie des Sciences. 224: 323–324.
↑Jabotinsky, Eri (1955). "Iterational invariants". Technion. Israel Institute of Technology. Scientific Publications. 6: 64–80.