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This paper describes a new manipulation technique for small-screen mobile devices. The proposed technique, called HybridTouch, uses a touchpad attached to the rear surface of a PDA. A user can manipulate the PDA by simultaneously touching the front surface with a stylus pen held by the dominant hand and the rear surface with a finger of the nondominant hand. User studies were conducted via applications augmented by HybridTouch, and proved that users could perform manipulation tasks intuitively.
Sugimoto et al. (Tue,) studied this question.