Summary: Molodtsov introduced the theory of soft sets, which can be seen as a new mathematical approach to vagueness. In this paper, we first point out that several assertions (Proposition 2.3 (iv)-
, Proposition 2.4 and Proposition 2.6 (iii), (iv)) in a previous paper by P.K. Maji, R. Biswas
and A.R. Roy
[Comput. Math. Appl. 45, No. 4–5, 555–562 (2003; Zbl 1032.03525
)] are not true in general, by counterexamples. Furthermore, based on the analysis of several operations on soft sets introduced in the same paper, we give some new notions such as the restricted intersection, the restricted union, the restricted difference and the extended intersection of two soft sets. Moreover, we improve the notion of complement of a soft set, and prove that certain De Morgan’s laws hold in soft set theory with respect to these new definitions.