Zhao, Lian Kuo, Product of Toeplitz operators on the harmonic Dirichlet space, Acta mathematica Sinica. English series (Print) , 28(5), 2012, pp. 1033-1040
In this paper, we study Toeplitz operators with harmonic symbols on the harmonic Dirichlet space, and show that the product of two Toeplitz operators is another Toeplitz operator only if one factor is constant