Fixed points in two metric spaces. Cho, Y.J.; Kang, S.M.; Kim, S.S. — 1999 Novi Sad Journal of Mathematics
Some Boas-Bellman type inequalities in 2-inner product spaces. Dragomir, S.S.; Cho, Y.J.; Kim, S.S.; Sofo, A. — 2005 JIPAM. Journal of Inequalities in Pure & Applied Mathematics [electronic only]