An extension of Hardy’s class L of ”orders of infinity”. (English) Zbl 0539.26002

Boshernitzan, Michael

J. Anal. Math. 39, 235-255 (1981).

Let B denote the ring of germs of continuous real-valued functions at infinity. There is a natural partial ordering on the ring. Two functions \(f,g\in B\) are called comparable if the difference (f-g) is ultimately \((=\) for all x sufficiently large) of definite sign and \(f\gg g\) if it is ultimately positive. This ordering plays an important role in the author’s considerations. Particularly, every subfied of B containing real constants only (only such fields are considered) is an ordered field with respect to the relation \(\gg\). Let \(C^ SB\), \(C^{\infty}B\) and \(C^ AB\) denote the differential subrings of B consisting of the sufficiently smooth (upper index means smoothness degree) functions at infinity. The intersection of all maximal differential subfields of such CB ring forms a differential field \(E=E(CB)\). The field E has a number of significant properties. For example, the author proved (Theorems 6.1; 6.8; 10.1) that:
(1) E is closed under composition of functions;
(2) each function \(f\in E\) is ultimately (real) analytic, satisfies an algebraic differential equation and moreover, there exists an iterate \(\exp(\exp(...\exp(x)...))=g(x)\) of \(e^ x\), such that \(g\gg f\);
(3) E is a real closed field;
(4) if \(f\in E\), then \(e^ f\in E\);
(5) if \(f\in E\) and \(f\neq 0\), then \(\ln | f| \in E\);
(6) if \(f\in E,\) then \(\int^{x}f(t)dt\in E;\)
(7) \(L\subset E\), where L denotes the class of Hardy’s L-functions [see G. H. Hardy, ”Orders of infinity” (Cambridge, 1924)]. The consideration of properties of E takes a central place in the paper and are applied to study the growth properties of real solutions of the differential equation \(y'=h(x,y)\) at infinity. For this equation the author obtains the results which are stronger and more general than the corresponding results of Hardy [see G. H. Hardy, Proc. Lond. Math. Soc., II. Ser. 10, 451-468 (1912)]. In conclusion the author formulated following conjectures: \(\quad E(C^ SB)=E(C^{\infty}B)=E(C^ AB);\) 2) E contains a differential closure of \({\mathbb{R}}\) in CB; 3) E is closed under compositional inversion.

Reviewer: N.V.Grigorenko

MSC:

26A12	Rate of growth of functions, orders of infinity, slowly varying functions
12H05	Differential algebra
34C11	Growth and boundedness of solutions to ordinary differential equations

Keywords:

maximal differential subfields; algebraic differential equation; Hardy’s L-functions; Orders of infinity; growth properties

Citations:

Zbl 0539.26003

References:

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