Bug in pushForward?

[Frank Moore]

Hello!

Consider the following example:

Q = QQ[t]

R = QQ[x,y,z,Degrees=>{3,5,7}]

phi = map(Q,R,matrix {{t^3,t^5,t^7}})

S = R/(ker phi)

psi = map(Q,S,matrix phi)

M = pushForward(psi,Q^1)

numgens M -- should be 4

M is just Q = QQ[t] viewed as a module over S = Q[t^3,t^5,t^7].  As an S-module, Q requires 4 generators, namely 1,t,t^2,t^4.  Therefore, numgens M should be 4.  M2 is only picking up the first three generators.  This seems to be a problem more generally with any subsemigroup of \bbN, in that M2 only returns the powers of t less than the lowest degree generator.

I suspect the offending command in the pushForward code is the line:

M2 := subquotient(matrix basis M1, relations M);

since matrix basis M1 is only returning the elements that are of small degree.  I'm not sure what command should go in its place.

Am I missing or not understanding something?

Frank

--
Dr. W. Frank Moore
Assistant Professor
Department of Mathematics, Wake Forest University

email: moo...@wfu.edu

[Frank Moore]

Disregard my previous email ;)  I of course don't need t^4, since t generates t^4 in this example.  Sorry!

[David Eisenbud]

Dear Frank,

I'd put this off to think about on the plane; thanks for the update!

David
--
David Eisenbud
Professor of Mathematics,
University of California, Berkeley
www.msri.org/~de

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