1. Almira, J.M., Luther, U.: Compactness and generalized approximation spaces. Numer. Funct. Anal. Optim. 23, 1–38 (2002)
2. Almira, J.M., Luther, U.: Generalized approximation spaces and applications. Math. Nachr. 263, 3–35 (2004)
3. Bingham, N.H., Goldie, C.M., Teugels, J.L.: Regular Variation, Encyclopedia of Mathematics and its Applications, vol. 27. Cambridge Univ. Press, Cambridge (1987)
4. Brudnyi, Ju.A., Krugljak, N.Ja.: A family of approximation spaces. In: Studies in the Theory of Functions of Several Real Variables, vol. 2, pp. 15–42. Yaroslav. Gos. Univ., Yaroslavl (1978) (in Russian)
5. Butzer, P.L., Scherer, K.: Approximationsprozesse und Interpolationsmethoden. Mannheim, Zürich (1968)