1. P. G. De Gennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974) p. 63.
2. J. Collings, Handbook of Liquid Crystal Research, eds. P. J. Collings and J. S. Patel (Oxford University Press, New York, London, 1997) p. 99.
3. S. A. Pikin, Structural Transformations in Liquid Crystals (Gordon and Breach, New York, 1991) p. 11.