TY  - RPRT
TI  - Small denominators, frequency operators, and Lie transforms for nearly integrable quantum spin systems.
AU  - T Gramespacher
AU  - S Weigert
PY  - 1996
DO  - 10.1103/physreva.53.2971
UR  - https://pubmed.ncbi.nlm.nih.gov/9913233/
ID  - 9913233
ER  -