1. [1] A. Boultif, & D. Louër, J. Appl. Cryst. 37, 724-731 (2004).
2. [2] J. H. Conway, “The Sensual (Quadratic) Form”, The mathematical association of America (1997).
3. [3] J. H. Conway & N. J. A. Sloane, Duke Math. J. 66, 93-96 (1992).
4. [4] M. Kac, Am. Math. Mon. 73, 1-23 (1966).
5. [5] T. Kobayashi, Jour. Fac. Sci. Univ. Tokyo 36 (3), 389-478 (1989).