Brizuela, B.M., & Earnest, D. (2008). Multiple notational systems and algebraic understandings: The case of the best deal problem. 

In J. J. Kaput, D. W. Carraher, & M. L. Blanton (Eds.), Algebra in the early grades (pp. 273-301). Mahwah, NJ: Lawrence Erlbaum Associates.