@article{lester_stone_stelling_1999, title={Lifelike pedagogical agents for mixed-initiative problem solving in constructivist learning environments}, volume={9}, ISSN={["1573-1391"]}, DOI={10.1023/A:1008374607830}, number={1-2}, journal={USER MODELING AND USER-ADAPTED INTERACTION}, author={Lester, JC and Stone, BA and Stelling, GD}, year={1999}, pages={1–44} }