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