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1J. Almeida and M. V. Volkov . The gap between partial and full. Int. J. Algebra Computation 8 (1998), 399–430.
2V. A. Gorbunov . Structure of lattices of varieties and lattices of quasivarieties: their similarity and difference. I. Algebra Logika 34 (1995), 142–168. (English transl. Algebra Logic 34 (1995), 73–86.)
3V. H. Fernandes . Semigroups of order preserving mappings on a finite chain: a new class of divisors. Semigroup Forum 54 (1997), 230–236.
4P. M. Higgins . Divisors of semigroups of order-preserving mappings on a finite chain. Int. J. Algebra Computation 5 (1995), 725–742.
5J. E. Pin . Varieties of formal languages (London: North Oxford Academic Publishers, 1986).