An oldie, but goodie! A classic piece in a publication called the constructivist, written by strings enthusiasts: DiBrienza, J., & Shevell, G. (1998). Number strings: Developing computational efficiency in a constructivist classroom. The Constructivist, 13(2), 21 –25.
An article on number strings by site founders Rachel and Kara.
Lambert, R., Imm, K., & Williams, D. A. (2017). Number Strings: Daily Computational Fluency. Teaching Children Mathematics, 24(1), 48–55.