Only two of the available symbols participate;
The symbol must appear exactly times and the symbol must appear exactly times and (the length of the word);
To ilustrate the problem suppose that the alphabet is and that I want words of length with two and two . With the stated restrictions all of the possible words are
So, I need to know what this is. Some references on aproaches on how to solve this kind of problems and related algorithms to generate such words would be apreciated.