Nil Şahin
Bilkent University, Ankara,Turkey

Let $S$ be a 4-generated pseudo symmetric semigroup generated by the positive integers $\{n_1,n_2,n_3,n_4 \}$ where $\gcd (n_1,n_2,n_3,n_4)=1$. $k$ being a field, let $k[S]$ be the corresponding semigroup ring and $I_S$ be the defining ideal of $S$. Tangent cone of $S$ is ${k[S]/I_S}_*$ where $I_S=< f_* | f \in I_S >$. We discuss homogeneity of $S$ when the tangent cone is Cohen-Macaulay.