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%I #29 Oct 02 2019 05:16:46
%S 1,2,4,8,15,30,56,107,200,372,680,1241,2221,3938,6880,11860,20148,
%T 33778,55814,91007,146392,232458,364462,564560,864230,1308160,1958700,
%U 2902417,4258009,6187350,8908680,12714829,17994860,25262928,35193094,48664341,66814595,91109910,123426848,166156091
%N Number of binary [ n,4 ] codes of dimension <= 4 without zero columns.
%C To get the g.f. of this sequence (with a constant 1), modify the Sage program below (cf. function f). It is too complicated to write it here. See the link below. - _Petros Hadjicostas_, Sep 30 2019
%H Discrete algorithms at the University of Bayreuth, <a href="http://www.algorithm.uni-bayreuth.de/en/research/SYMMETRICA/">Symmetrica</a>. [This package was used by Harald Fripertinger to compute T_{nk2} = A076832(n,k) using the cycle index of PGL_k(2). Here k = 4. That is, a(n) = T_{n,4,2} = A076832(n,4), but we start at n = 1 rather than at n = 4.]
%H Harald Fripertinger, <a href="http://www.mathe2.uni-bayreuth.de/frib/codes/tables.html">Isometry Classes of Codes</a>.
%H Harald Fripertinger, <a href="http://www.mathe2.uni-bayreuth.de/frib/codes/tables_3.html">Tnk2: Number of the isometry classes of all binary (n,r)-codes for 1 <= r <= k without zero-columns</a>. [This is a rectangular array whose lower triangle is A076832(n,k). Here we have column k = 4.]
%H Harald Fripertinger, <a href="https://imsc.uni-graz.at/fripertinger/codes_bms.html">Enumeration of isometry classes of linear (n,k)-codes over GF(q) in SYMMETRICA</a>, Bayreuther Mathematische Schriften 49 (1995), 215-223. [See pp. 216-218. A C-program is given for calculating T_{nk2} in Symmetrica. Here k = 4.]
%H Harald Fripertinger, <a href="https://doi.org/10.1016/S0024-3795(96)00530-7">Cycle of indices of linear, affine, and projective groups</a>, Linear Algebra and its Applications 263 (1997), 133-156. [See p. 152 for the computation of T_{nk2} = A076832(n,k). Here k = 4.]
%H H. Fripertinger and A. Kerber, <a href="https://doi.org/10.1007/3-540-60114-7_15">Isometry classes of indecomposable linear codes</a>. In: G. Cohen, M. Giusti, T. Mora (eds), Applied Algebra, Algebraic Algorithms and Error-Correcting Codes, 11th International Symposium, AAECC 1995, Lect. Notes Comp. Sci. 948 (1995), pp. 194-204. [The notation for A076832(n,k) is T_{nk2}. Here k = 4.]
%H Petros Hadjicostas, <a href="/A034338/a034338.txt">Generating function for a(n)</a>.
%H David Slepian, <a href="https://archive.org/details/bstj39-5-1219">Some further theory of group codes</a>, Bell System Tech. J. 39(5) (1960), 1219-1252.
%H David Slepian, <a href="https://doi.org/10.1002/j.1538-7305.1960.tb03958.x">Some further theory of group codes</a>, Bell System Tech. J. 39(5) (1960), 1219-1252.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Cycle_index">Cycle index</a>.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Projective_linear_group">Projective linear group</a>.
%o (Sage) # Fripertinger's method to find the g.f. of column k for small k:
%o def Tcol(k, length):
%o G = PSL(k, GF(2))
%o D = G.cycle_index()
%o f = sum(i[1]*prod(1/(1-x^j) for j in i[0]) for i in D)
%o return f.taylor(x, 0, length).list()
%o # For instance the Taylor expansion for column k = 4 gives a(n):
%o print(Tcol(4, 30)) # _Petros Hadjicostas_, Sep 30 2019
%Y Column k=4 of A076832 (starting at n=4).
%Y Cf. A034337.
%K nonn
%O 1,2
%A _N. J. A. Sloane_.
%E More terms from _Petros Hadjicostas_, Sep 30 2019
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