$Z_2$ Symmetry of $AdS_3 \times S^3 \times S^3 \times S^1$ Superstrings

Abstract: We discuss the importance of $Z_2$ symmetry of large $N=4$ superconformal algebra for finding its free field realization that is more complete than the most commonly used one, that is, the Sevrin, Troost and van Proeyen realization. This in turn has implications for the search of a conformal field theory dual to superstrings moving in $AdS_3 \times S^3 \times S^3 \times S^1$ background.