0,2

H. Brown, R. B\"{u}low, J. Neub\"{u}ser, H. Wondratschek and H. Zassenhaus, Crystallographic Groups of Four-Dimensional Space. Wiley, NY, 1978, p. 52.

P. Engel, ``Geometric crystallography,'' in P. M. Gruber and J. M. Wills, editors, Handbook of Convex Geometry. North-Holland, Amsterdam, Vol. B, pp. 989-1041.

J. E. Goodman and J. O'Rourke, editors, Handbook of Discrete and Computational Geometry, CRC Press, 1997, p. 102 and 934.

T. Janssen, Crystallographic Groups. North-Holland, Amsterdam, 1973, p. 119.

W. Plesken and T. Schulz, Counting crystallographic groups in low dimensions, Experimental Mathematics, 9 (No. 3, 2000), 407-411.

R. L. E. Schwarzenberger, $N$-Dimensional Crystallography. Pitman, London, 1980, p. 34.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Dror Bar-Natan, Illustrations of 2-dimensional symmetry groups

W. Plesken and T. Schulz, CARAT Homepage

Experimental Mathematics, Home Page

Cf. A006227, A004027.

Sequence in context: A102023 A058010 A126752 this_sequence A114268 A144208 A058239

Adjacent sequences: A004026 A004027 A004028 this_sequence A004030 A004031 A004032

nonn,hard,nice

N. J. A. Sloane (njas(AT)research.att.com).

a(7) corrected by W. Plesken and T. Schulz. Thanks to Max Horn for reporting this correction, Dec 18 2009