piF = directImageComplex F
We give an application of this function to create pure free resolutions. A much more general tool for doing this is in the script pureResolution.
A "pure free resolution of type (d_0,d_1,..,d_n)" is a resolution of a graded Cohen-Macaulay module M over a polynomial ring such that for each i = 1,..,n, the module of i-th syzygies of M is generated by syzygies of degree d_i. Eisenbud and Schreyer constructed such free resolutions in all characteristics and for all degree sequences $d_0 < d_1 < \cdots < d_n$ by pushing forward appropriate twists of a Koszul complex. (The construction was known for the Eagon-Northcott complex since work of Kempf).
If one of the differences $d_{i+1} - d_i$ is equal to 1, then it turns out that one of the maps in the pure resolution is part of the map of complexes directImageComplex k_j, where k_j is a map in this Koszul complex. Here is a simple example, where we produce one of the complexes in the family that included the Eagon-Northcott complex (see for example "Commutative Algebra with a View toward Algebraic Geometry", by D. Eisenbud.)
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The direct image complexes each have only one nonzero term,and so D has only one nonzero component. According to Eisenbud and Schreyer, this is the last map in a pure resolution. Since the dual of a pure resolution is again a resolution, we can simply take the dual of this map and resolve to see the dual of the resolution (or dualize again to see the resolution itself, which is the Eagon-Northcott complex itself in this case.
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