Simplicial Resolutions of the Quadratic Power of Monomial Ideals

Given any monomial ideal $I$ minimally generated by $q$ monomials, we define a simplicial complex $\mathbb{M}_q^2$ that supports a resolution of $I^2$. We also define a subcomplex $\mathbb{M}^2(I)$, which depends on the monomial generators of $I$ and also supports the resolution of $I^2$. As a byproduct, we obtain bounds on the projective dimension of the second power of any monomial ideal. We also establish bounds on the Betti numbers of $I^2$, which are significantly tighter than those determined by the Taylor resolution of $I^2$. Moreover, we introduce the permutation ideal $\mathcal{T}_q$ which is generated by $q$ monomials. For any monomial ideal $I$ with $q$ generators, we establish that $\beta(I^2) \leq \beta({\mathcal{T}_q}^2)$. We show that the simplicial complex $\mathbb{M}_q^2$ supports the minimal resolution of ${\mathcal{T}_q}^2$. In fact, $\mathbb{M}_q^2$ is the Scarf complex of ${\mathcal{T}_q}^2$.