Extensions of positive operators and extreme points. I Z. Lipecki, D. Plachky, W. Thomsen (1979) Colloquium Mathematicae
Extensions of positive operators and extreme points. IV Z. Lipecki, W. Thomsen (1982) Colloquium Mathematicae
Extremal subobjects of complete posets Ana Pasztor (1986) Commentationes Mathematicae Universitatis Carolinae
Extremal units in an archimedean Riesz space Anthony W. Hager, Lewis C. Robertson (1978) Rendiconti del Seminario Matematico della Università di Padova