Monic modules and semi-Gorenstein-projective modules

Abstract: The category ${\rm gp}(\Lambda)$ of Gorenstein-projective modules over tensor algebra $\Lambda = A\otimes_kB$ can be described as the monomorphism category ${\rm mon}(B, {\rm gp}(A))$ of $B$ over ${\rm gp}(A)$. In particular, Gorenstein-projective $\Lambda$-modules are monic. In this paper, we find the similar relation between semi-Gorenstein-projective $\Lambda$-modules and $A$-modules, via monic modules, namely, ${\rm mon}(B, \ ^\perp A) = {\rm mon}(B, A)\cap \ ^\perp \Lambda.$ Using this, it is proved that if $A$ is weakly Gorenstein, then $\Lambda$ is weakly Gorenstein if and only each semi-Gorenstein-projective $\Lambda$-modules are monic; and that if $B = kQ$ with $Q$ a finite acyclic quiver, then $\Lambda$ is weakly Gorenstein if and only if $A$ is weakly Gorenstein. However, this relation itself does not answer the question whether there exist double semi-Gorenstein-projective $\Lambda$-modules which are not monic. Using the recent discovered examples of double semi-Gorenstein-projective $A$-modules which are not torsionless, we positively answer this question, by explicitly constructing a class of double semi-Gorenstein-projective $T_2(A)$-modules with one parameter such that they are not monic, and hence not torsionless. The corresponding results are obtained also for the monic modules and semi-Gorenstein-projective modules over the triangular matrix algebras given by bimodules.