OFFSET
0,2
REFERENCES
Inorganic Crystal Structure Database: Collection Code 29343
Michael O'Keeffe, Topological and geometrical characterization of sites in silicon carbide polytypes, Chemistry of Materials 3 (2) (1991), 332-335. (Eq. (2) gives an empirical formula for a(n). - N. J. A. Sloane, Apr 07 2018)
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
M. L. Glasser, Symmetry properties of the wurtzite structure, Journal of Physics and Chemistry of Solids, 10(2-3) (1959), 229-241.
Ralf W. Grosse-Kunstleve, Zeolites, Frameworks, Coordination Sequences and Encyclopedia of Integer Sequences, 1996.
Ralf W. Grosse-Kunstleve, G. O. Brunner and N. J. A. Sloane, Algebraic Description of Coordination Sequences and Exact Topological Densities for Zeolites, Acta Cryst., A52 (1996), 879-889.
Sean A. Irvine, Generating Functions for Coordination Sequences of Zeolites, after Grosse-Kunstleve, Brunner, and Sloane.
Michael O'Keeffe, N-dimensional diamond, sodalite and rare sphere packings, Acta Cryst. A 47 (1991), 749-753.
Michael O'Keeffe, Topological and geometrical characterization of sites in silicon carbide polytypes, Chemistry of Materials 3 (2) (1991), 332-335.
Reticular Chemistry Structure Resource (RCSR), The lon net (lonsdaleite) and The lon-b net (wurtzite).
Index entries for linear recurrences with constant coefficients, signature (2,-1,0,1,-2,1).
FORMULA
a(4*m+k) = 42*m^2 + 21*k*m + [ 2, 4, 12, 25 ], 0 <= k < 4 (N. J. A. Sloane).
a(n) = 1 + (42*n^2 + (1 + (-1)^n)*(3 + 2*(-1)^((n - 1)*n/2)) + 6)/16 for n > 0, a(0) = 1. - Bruno Berselli, Jul 24 2013
G.f.: (1 + 2*x + 5*x^2 + 5*x^3 + 5*x^4 + 2*x^5 + x^6)/((1 - x)^3*(1 + x + x^2 + x^3)). - Bruno Berselli, Jul 24 2013
MATHEMATICA
a[n_] := (m = Quotient[n, 4]; k = Mod[n, 4]; 42*m^2 + 21*k*m + Switch[k, 0, 2, 1, 4, 2, 12, 3, 25]); a[0]=1; Table[a[n], {n, 0, 47}] (* Jean-François Alcover, Oct 11 2012, from the first formula *)
Join[{1}, Table[1 + (42 n^2 + (1 + (-1)^n) (3 + 2 (-1)^((n - 1) n/2)) + 6)/16, {n, 50}]] (* Bruno Berselli, Jul 24 2013 *)
LinearRecurrence[{2, -1, 0, 1, -2, 1}, {1, 4, 12, 25, 44, 67, 96}, 20] (* Harvey P. Dale, Dec 27 2016 *)
PROG
(PARI) a(n)=if(n, 1+(42*n^2+(1+(-1)^n)*(3+2*(-1)^((n-1)*n/2))+6)/16, 1) \\ Charles R Greathouse IV, Feb 10 2017
CROSSREFS
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved