Juhani Karhumäki Wojciech Plandowski On the Size of Independent Systems of Equations in Semigroups. 105-119 1996 168 Theor. Comput. Sci. 1 db/journals/tcs/tcs168.html#KarhumakiP96 http://dx.doi.org/10.1016/S0304-3975(96)00064-3