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## Studia Mathematica

2007 | 178 | 2 | 99-124
Tytuł artykułu

### Minimal multi-convex projections

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EN
We say that a function from $X = C^{L}[0,1]$ is k-convex (for k ≤ L) if its kth derivative is nonnegative. Let P denote a projection from X onto V = Πₙ ⊂ X, where Πₙ denotes the space of algebraic polynomials of degree less than or equal to n. If we want P to leave invariant the cone of k-convex functions (k ≤ n), we find that such a demand is impossible to fulfill for nearly every k. Indeed, only for k = n-1 and k = n does such a projection exist. So let us consider instead a more general "shape" to preserve. Let σ = ( σ₀, σ₁, ..., σₙ) be an (n + 1)-tuple with $σ_{i} ∈ {0, 1}$; we say f ∈ X is multi-convex if $f^{(i)} ≥ 0$ for i such that $σ_{i} = 1$. We characterize those σ for which there exists a projection onto V preserving the multi-convex shape. For those shapes able to be preserved via a projection, we construct (in all but one case) a minimal norm multi-convex preserving projection. Out of necessity, we include some results concerning the geometrical structure of $C^{L}[0,1]$.
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Tom
Numer
Strony
99-124
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wydano
2007
Twórcy
autor
• Department of Mathematics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland
autor
• Department of Mathematics, University of Northern Iowa, Cedar Falls, IA 50614-0506, U.S.A.
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