@article{Das_j2004_e, author = {J{\"u}rgen~Dassow and Victor~Mitrana and Gheorghe~P\u{a}un and Ralf~Stiebe}, title = {On differentiation functions, structure functions, and related languages of context-free grammars}, journal = rairoita, volume = 38, number = 3, pages = {257--267}, year = 2004, url = {http://www.edpsciences.org/articles/ita/abs/2004/03/ita0408/ita0408.html} }