returns the preparation equation of a differential polynomial
PreparationEquation (f, regchain, opts)
a differential polynomial
a regular differential chain
a sequence of options
The opts arguments may contain one or more of the options below.
congruence = true. In the right hand-side of the returned preparation equation, denote q the minimum total degree of the monomials ti. All the terms ci⁢ti such that the total degree of ti is greater than q, are removed from the right hand-side of the preparation equation. This stripped preparation equation is called a preparation congruence of f.
n = nonnegative (default value is 0). This option is useful in conjunction with the option congruence = true. The n first differential polynomials A1, ..., An of regchain are considered as equations defining the base field of f, and, of the differential polynomials An+1, ..., Ar. Reductions by the base field equations are not taken into account for computing the preparation congruence of f: the terms ti involving derivatives of z1, ..., zn are not considered for determining q, and, do not appear in the preparation congruence.
zstring = string. This option permits to customize the identifier used for the new variables zk. It must be a valid MAPLE identifier (possibly an indexed) involving the substring "%d".
notation = jet, tjet, diff or Diff. Specifies the notation used for the result of the function call. If not specified, the notation of regchain is used.
memout = nonnegative. Specifies a memory limit, in MB, for the computation. Default is zero (no memory out).
The function call PreparationEquation (f, regchain) returns a preparation equation [K73, chapter IV, section 13] for f with respect to regchain. The argument f is regarded as a differential polynomial of the embedding ring of regchain.
Let I denote the differential ideal defined by regchain and denote A1, ..., Ar the differential polynomials which constitute the chain. Introduce r new dependent variables zi. Each variable zi represents the differential polynomial Ai.
The returned preparation equation is an expression having the form h⁢f = c1⁢t1 + ... + cn⁢tn. The differential polynomial h is a power product of initials and separants of the Ak. The coefficients ci are reduced and regular with respect to I. The monomials ti are power products of the zk variables and their derivatives. They satisfy some further properties, described in [K73, chapter IV, section 13]. If each zk is replaced by the corresponding Ak, in all terms ti, then the preparation equation becomes a true equality.
Preparation equations are an important tool in the context of the Low Power Theorem. See RosenfeldGroebner with the option singsol = essential.
This command is part of the DifferentialAlgebra:-Tools package. It can be called using the form PreparationEquation(...) after executing the command with(DifferentialAlgebra:-Tools). It can also be directly called using the form DifferentialAlgebra[Tools][PreparationEquation](...).
The following examples illustrate the function, syntactically.
R ≔ DifferentialRing⁡derivations=t,blocks=u,s,c
ideal ≔ PretendRegularDifferentialChain⁡c2+s2−1,ct+s,u,R,pretend=false
f ≔ ct+s⁢ut+ut2
prepeq ≔ PreparationEquation⁡f,ideal
If one substitutes the Ai to the zi, the equation becomes an equality.
Changing the identifier for the zi.
Since all monomials ti have degree q=2, the preparation congruence is equal to the preparation equation.
However, if the two first elements of the regular differential chain are considered as base field defining equations, then, only one monomial ti is left in the congruence.
The Low Power Theorem
The next example illustrates the Low Power Theorem. See [R50, chapter III] and [K73, chapter IV, section 15].
R ≔ DifferentialRing⁡derivations=t,blocks=y
f ≔ yt3−4⁢yt⁢y⁢t+8⁢y2
First compute a representation of the radical of the differential ideal generated by f, by means of RosenfeldGroebner.
ideal ≔ RosenfeldGroebner⁡f,R
Second, remove any regular differential chain which involve two or more differential polynomials, by application of the Component Theorem [K73, chapter IV, section 14]. In our case, no regular differential chain is removed by this process. Third, compute a preparation congruence for f, with respect to each of the two singular components, i.e., the two last ones.
In the first case, there is only one monomial t1, of the form z1q. Thus this regular differential chain must be kept in the decomposition.
In the second case, the right hand-side of the preparation congruence involves two monomials. Thus this regular differential chain is redundant.
Indeed, RosenfeldGroebner with the option singsol = essential removes the second singular component from the decomposition.
ideal ≔ RosenfeldGroebner⁡f,R,singsol=essential
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