Boggess A., R.C. Bohlin, D.C. Evans, H.R. Freeman, T.R. Gull, S.R. Heap, D.A. Klinglesmith, G.R. Longanecker, W. Sparks, D.K. West, and A.V. Holm, Nature 275, 377-385 (1978).
Reichert G.A., P.M. Rodriquez-Pascual, D. Alloin, J. Clavel, D.M. Crenshaw, G.A. Kriss, J.H. Krolik, M.A. Malkan, H. Netzer, B.M. Peterson, and W.M. Wamsteker, The Astrophysical Journal 425, 582 (1994).
and 
