Thursday, 16 September 2010

ag.algebraic geometry - In what degrees does Ext(S/(f),S) vanish?

Let S=k[x0,...,xn] be the polynomial ring over a field k and finS non-zero and homogeneous. Is it true that Extm(S/(f),S) is zero?



This would help me to show that Extm(S/fI,S)congExtm(S/I,S)(degf) for mgeq2 and a homogeneous ideal I of codim geq2. I tried the following approach:
Applying the long exact sequence of Ext to the exact sequence of graded S-modules
0toS/IxrightarrowcdotfS/fI(degf)xrightarrowtauS/(f)(degf)to0,


where tau is the canonical morphism s+fImapstos+(f),
brings
ldotstoExtm(S/(f)(degf),S)toExtm(S/fI(degf),S)toExtm(S/I,S)to

Extm+1(S/(f)(degf),S)toExtm+1(S/fI(degf),S)toExtm+1(S/I,S)toldots

and two zeros on the left would suffice.

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