English

The resolution of the bracket powers of the maximal ideal in a diagonal hypersurface ring

Commutative Algebra 2010-12-07 v1

Abstract

Let kk be a field. For each pair of positive integers (n,N)(n,N), we resolve Q=R/(xN,yN,zN)Q=R/(x^N,y^N,z^N) as a module over the ring R=k[x,y,z]/(xn+yn+zn)R=k[x,y,z]/(x^n+y^n+z^n). Write NN in the form N=an+rN=a n+r for integers aa and rr, with rr between 00 and n1n-1. If nn does not divide NN and the characteristic of kk is fixed, then the value of aa determines whether QQ has finite or infinite projective dimension. If QQ has infinite projective dimension, then value of rr, together with the parity of aa, determines the periodic part of the infinite resolution. When QQ has infinite projective dimension we give an explicit presentation for the module of first syzygies of QQ. This presentation is quite complicated. We also give an explicit presentation the module of second syzygies for QQ. This presentation is remarkably uncomplicated. We use linkage to find an explicit generating set for the grade three Gorenstein ideal (xN,yN,zN):(xn+yn+zn)(x^N,y^N,z^N):(x^n+y^n+z^n) in the polynomial ring k[x,y,z]k[x,y,z]. The question "Does QQ have finite projective dimension?" is intimately connected to the question "Does k[X,Y,Z]/(Xa,Ya,Za)k[X,Y,Z]/(X^a,Y^a,Z^a) have the Weak Lefschetz Property?". The second question is connected to the enumeration of plane partitions. When the field kk has positive characteristic, we investigate three questions about the Frobenius powers Ft(Q)F^t(Q) of QQ. When does there exist a pair (n,N)(n,N) so that QQ has infinite projective dimension and F(Q)F(Q) has finite projective dimension? Is the tail of the resolution of the Frobenius power Ft(Q)F^t(Q) eventually a periodic function of tt, (up to shift)? In particular, we exhibit a situation where the tail of the resolution of Ft(Q)F^t(Q), after shifting, is periodic as a function of tt, with an arbitrarily large period. Can one use socle degrees to predict that the tail of the resolution of Ft(Q)F^t(Q) is a shift of the tail of the resolution of QQ?

Keywords

Cite

@article{arxiv.1012.1026,
  title  = {The resolution of the bracket powers of the maximal ideal in a diagonal hypersurface ring},
  author = {Andrew R. Kustin and Hamid Rahmati and Adela Vraciu},
  journal= {arXiv preprint arXiv:1012.1026},
  year   = {2010}
}