On Isbell's Density Theorem for bitopological pointfree spaces I

This paper addresses dense sub-objects for point-free bitopology in terms of $d$-frames and provides several examples. We characterize extremal epimorphisms in $d$-frames and show that a smallest dense one always exists, establishing a proper analogue of Isbell's Density Theorem for $d$-frames. Further we explore certain questions about the functoriality of assigning the smallest dense sub-object to each pointfree bitopological space.