DELTA OPERATOR

In mathematics, a 'delta operator' is a shift-equivariant linear operator ''$scriptstyle\{\; Q:mathbb\; K[x]\; longrightarrow\; mathbb\; K[x]\; \}$'' on the vector space of polynomials in a variable $scriptstyle\; x$ over a field $scriptstyle\{\; mathbb\; K\}$ that reduces degrees by one.
To say that $scriptstyle\; Q$ is 'shift-equivariant' means that if $scriptstyle\{\; g(x)\; =\; f(x\; +\; a)\}$, then
:$\{\; (Qg)(x)\; =\; (Qf)(x\; +\; a)\}.,$
In other words, if ''$f$'' is a "'shift'" of ''$g$'', then ''$Qf$'' is also a shift of ''$Qg$'', and has the same "'shifting vector'" ''$a$''.
To say that ''an operator reduces degree by one'' means that if ''$f$'' is a polynomial of degree ''$n$'', then ''$Qf$'' is either a polynomial of degree $n-1$, or, in case $n\; =\; 0$, ''$Qf$'' is 0.
Sometimes a ''delta operator'' is defined to be a shift-equivariant linear transformation on polynomials in ''$x$'' that maps ''$x$'' to a nonzero constant. Seemingly weaker than the definition given above, this latter characterization can be shown to be equivalent to the stated definition, since shift-equivariance is a fairly strong condition.

Contents

Examples

Basic polynomials

See also

References

Examples

★ The forward difference operator
:: $(Delta\; f)(x)\; =\; f(x\; +\; 1)\; -\; f(x),$
:is a delta operator.

★ Differentiation with respect to ''$x$'', written as ''$D$'', is also a delta operator.

★ More generally the ''n^th'' derivative $scriptstyle\; D^n(f)\; =\; f^\{(n)\}$ is a shift-equivariant operator which reduces the degree by ''$n$''. The sum
::$\{\; Q\; =\; sum\_\{n=1\}^infty\; a\_n\; D^n\; \}$
:is then a shift-equivariant operator: to be a delta operator, it needs only to decreases the degree by one, that is to have $scriptstyle\{\; a\_0=0\; \}$ and $scriptstyle\{\; a\_1$
ot = 0}.
:It can be shown that there are no other delta operators. For example, a Taylor series expansion shows that the difference operator given above can be written as
::.

★ In computer science and cybernetics, the term "discrete-time delta operator" (δ) is generally taken to mean a difference operator
:: $\{(delta\; f)(x)\; =\; \{\{\; f(x+Delta\; t)\; -\; f(x)\; \}\; over\; \{Delta\; t\}\; \}\},$
: the Euler approximation of the usual derivative with a discrete sample time $Delta\; t$. The delta-formulation obtains a significant number of numerical advantages compared to the shift-operator at fast sampling.

Basic polynomials

Every delta operator ''$Q$'' has a unique sequence of "basic polynomials", a polynomial sequence defined by three conditions:

★ $scriptstyle\; p\_0(x)=1\; ;$

★ $scriptstyle\; p\_\{n\}(0)=0;$

★
Such a sequence of basic polynomials is always of binomial type, and it can be shown that no other sequences of binomial type exist. If the first two conditions above are dropped, then the third condition says this polynomial sequence is a Sheffer sequence -- a more general concept.

See also

★ Pincherle derivative

★ Shift operator

★ Umbral calculus

References

★ ISBN 0-387-15021-8.