Although not a mathematics education researcher, Lee Shulman's ideas about pedagogical content knowledge have influenced the mathematics education community and the conceptualization of mathematical knowledge for teaching (Hill, Schilling, & Ball, 2004; Hill, Rowan, & Ball, 2005; Ball, Thames, & Phelps, 2008).
Shulman, L. S. (1986). Those who understand: Knowledge growth in teaching. Educational Researcher, 15(2), 4–14. doi:10.3102/0013189X015002004