1. D. Shoemaker, D. Coyne, LIGO II Seismic Isolation Design Requirements Document, LIGO internal document LIGO-E990304-01.
2. G. Cella, R. de Salvo, V. Sannibale, A. Takamori, H. Tariq, C. Wang, Performances of the First Prototype of Geometric Anti-spring Filter For Seismic Attenuation, LIGO Project internal document LIGO-T000103-00-R (1999), to be submitted for publication.
3. D. Passuello, INFN Sezione di Pisa ,Via Livornese 1291, 56010 S. Piero a Grado, Italy.