THE CLEANNESS OF (SYMBOLIC) POWERS OF STANLEY-REISNER IDEALS

Let Δ be a pure simplicial complex on the vertex set [n] = {1,..., n} and I Δ its Stanley-Reisner ideal in the polynomial ring S = K[x 1,..., x n]. We show that Δ is a matroid (complete intersection) if and only if S/I Δ (m) (S/I Δ (m)) is clean for all m ∈ N and this is equivalent to saying that S/I Δ (m) (S/I Δ (m), respectively) is Cohen-Macaulay for all m ∈ N. By this result, we show that there exists a monomial ideal I with (pretty) cleanness property while S/I m or S/I m is not (pretty) clean for all integer m ≥ 3. If dim(Δ) = 1, we also prove that S/I Δ (2) Δ (S/I Δ 2) is clean if and only if S/I Δ (2) (S/I Δ 2, respectively) is Cohen-Macaulay.