Deviation of a local ring

In commutative algebra, the deviations of a local ring R are certain invariants ε_i(R) that measure how far the ring is from being regular.

Definition

The deviations ε_n of a local ring R with residue field k are non-negative integers defined in terms of its Poincaré series P(t) by

P(t)=\sum _{n\geq 0}t^{n}\operatorname {Tor} _{n}^{R}(k,k)=\prod _{n\geq 0}{\frac {(1+t^{2n+1})^{\varepsilon _{2n}}}{(1-t^{2n+2})^{\varepsilon _{2n+1}}}}.

The zeroth deviation ε₀ is the embedding dimension of R (the dimension of its tangent space). The first deviation ε₁ vanishes exactly when the ring R is a regular local ring, in which case all the higher deviations also vanish. The second deviation ε₂ vanishes exactly when the ring R is a complete intersection ring, in which case all the higher deviations vanish.

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References

Gulliksen, T. H. (1971), "A homological characterization of local complete intersections", Compositio Mathematica, 23: 251–255, ISSN 0010-437X, MR 0301008

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