1. M. Ward, The closure operators of a lattice. Ann. Math.43, 191?196 (1942).
2. Ph. Dwinger, On the closure operators of a complete lattice. K. Proc. Kon. Ned. Akad. Wetensch. A57, 560?563 (1954).
3. Ph. Dwinger, On the closure operators of the ordinal product of closed lattices. K. Proc. Kon. Ned. Akad. Wetensch. A58, 36?40 (1955).
4. Ph. Dwinger, The closure operators of the cardinal and ordinal sums and products of partially ordered sets and closed lattices. K. Proc. Kon. Ned. Akad. Wetensch. A58, 341?351 (1955).
5. G.Birkhoff, Lattice Theory. Amer. Math. Soc. Colloq. Publications, Vol. 25, 1948.