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On a modified Hyers-Ulam sequence. (English) Zbl 0746.46038
It is shown that, given a mapping $$f: E\to F$$ between Banach spaces, which is continuous along rays through 0, and for which there is a $$\delta\geq 0$$ and a $$0\leq p<1$$ such that $\| f(x+y)-f(x)- f(y)\|\leq\delta\cdot(\| x\|^ p+\| y\|^ p)\text{ for all } x,y,$ there is a unique linear mapping $$T: E\to F$$ such that for all integers $$k\geq 2$$ one has $\| f(x)-T(x)\|\leq\delta\cdot\| x\|^ p\cdot {{k+\sum_{j=2}^{k-1}j^ k} \over {k-k^ p}} \text{ for all } x.$ In fact $$T(x)=\lim_{n\to\infty} {{f(k^ n x)}\over{k^ n}}$$. The author poses the question whether the minimum is attained for $$k=2$$.
Reviewer: A.Kriegl (Wien)

##### MSC:
 46G05 Derivatives of functions in infinite-dimensional spaces 41A35 Approximation by operators (in particular, by integral operators)
##### Keywords:
Gâteaux-derivative; approximation by linear operators
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##### References:
 [1] Hyers, D.H, On the stability of the linear functional equation, (), 222-224 · Zbl 0061.26403 [2] Hyers, D.H, The stability of homomorphisms and related topics, (), 140-153 · Zbl 0517.22001 [3] Rassias, Th.M, On the stability of the linear mapping in Banach spaces, (), 297-300 · Zbl 0398.47040 [4] Ulam, S.M, Problems in modern mathematics, (), Chap. VI · Zbl 0137.24201 [5] Ulam, S.M, Sets, numbers, and universes. selected works, (1974), MIT Press Cambridge, MA, Part III
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