1. CJM 2007 (vol 59 pp. 638)
|Distance from Idempotents to Nilpotents |
We give bounds on the distance from a non-zero idempotent to the set of nilpotents in the set of $n\times n$ matrices in terms of the norm of the idempotent. We construct explicit idempotents and nilpotents which achieve these distances, and determine exact distances in some special cases.
Keywords:operator, matrix, nilpotent, idempotent, projection
Categories:47A15, 47D03, 15A30
2. CJM 2000 (vol 52 pp. 197)
|Sublinearity and Other Spectral Conditions on a Semigroup |
Subadditivity, sublinearity, submultiplicativity, and other conditions are considered for spectra of pairs of operators on a Hilbert space. Sublinearity, for example, is a weakening of the well-known property~$L$ and means $\sigma(A+\lambda B) \subseteq \sigma(A) + \lambda \sigma(B)$ for all scalars $\lambda$. The effect of these conditions is examined on commutativity, reducibility, and triangularizability of multiplicative semigroups of operators. A sample result is that sublinearity of spectra implies simultaneous triangularizability for a semigroup of compact operators.
Categories:47A15, 47D03, 15A30, 20A20, 47A10, 47B10
3. CJM 1998 (vol 50 pp. 929)
|Decomposition varieties in semisimple Lie algebras |
The notion of decompositon class in a semisimple Lie algebra is a common generalization of nilpotent orbits and the set of regular semisimple elements. We prove that the closure of a decomposition class has many properties in common with nilpotent varieties, \eg, its normalization has rational singularities. The famous Grothendieck simultaneous resolution is related to the decomposition class of regular semisimple elements. We study the properties of the analogous commutative diagrams associated to an arbitrary decomposition class.
Categories:14L30, 14M17, 15A30, 17B45