## Abstract

A relatively compressed algebra with given socle degrees is an Artinian quotient A of a given graded algebra R/c, whose Hilbert function is maximal among such quotients with the given socle degrees. For us c is usually a "general" complete intersection and we usually require that A be level. The precise value of the Hilbert function of a relatively compressed algebra is open, and we show that finding this value is equivalent to the Fröberg conjecture. We then turn to the minimal free resolution of a level algebra relatively compressed with respect to a general complete intersection. When the algebra is Gorenstein of even socle degree we give the precise graded Betti numbers. When it is of odd socle degree we give good bounds on the graded Betti numbers. We also relate this case to the Minimal Resolution Conjecture of Mustaţǎ for points on a projective variety. Finding the graded Betti numbers is essentially equ ivalent to determining to what extent there can be redundant summands (i.e., "ghost terms") in the minimal free resolution, i.e., when copies of the same R(-t) can occur in two consecutive free modules. This is easy to arrange using Koszul syzygies; we show that it can also occur in more surprising situations that are not Koszul. Using the equivalence to the Fröberg conjecture, we show that in a polynomial ring where that conjecture holds (e.g., in three variables), the possible non-Koszul ghost terms are extremely limited. Finally, we use the connection to the Fröberg conjecture, as well as the calculation of the minimal free resolution for relatively compressed Gorenstein algebras, to find the minimal free resolution of general Artinian almost complete intersections in many new cases. This greatly extends previous work of the first two authors.

Original language | English |
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Pages (from-to) | 333-370 |

Number of pages | 38 |

Journal | Journal of Algebra |

Volume | 284 |

Issue number | 1 |

DOIs | |

State | Published - Feb 1 2005 |

### Bibliographical note

Funding Information:The authors thank the Mathematical Research Institute at Oberwolfach, where the authors first discussed this problem. Part of the work for this paper was done while the first author was sponsored by the National Security Agency under grant no. MDA904-03-1-0071. The second author was partially sponsored by the grant BFM2001-3584. The authors thank Tony Iarrobino for helpful comments about the exposition of this paper and the connections to his work.

## ASJC Scopus subject areas

- Algebra and Number Theory