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 A269455 Number of Type I (singly-even) self-dual binary codes of length 2n. 1
 1, 3, 15, 105, 2295, 75735, 4922775, 625192425, 163204759575, 83724041661975, 85817142703524375, 175667691114114395625, 720413716161839357604375, 5902349576513949856852644375, 96709997811181068404530578084375, 3168896498278970068411253452090715625, 207692645973961964120828372930661061284375, 27222898185745116523209337325140537285726884375, 7136346644902153570976711733098966146766874104484375, 3741493773415815389266667264411257664189964123617799515625 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A self dual binary linear code is either Type I (singly even) or Type II (doubly even).  A self dual binary linear code can only be Type II if the length of the code (2n) is a multiple of 8. The total number self dual binary linear codes (including equivalent codes) is equal to the number of Type I self dual binary linear codes (including equivalent codes) when the length (2n) is not a multiple of 8.  If the length is a multiple of 8 ( 2n =0 mod 8 ) then the total number of  Type I codes is the number of type II codes subtracted from the total number of self dual codes of length 2n. REFERENCES W. Cary Huffman and Vera Pless, Fundamentals of Error Correcting Codes, 2003, Page 366. F. J. MacWilliams and N. J. A. Sloane, The Theory of Error-Correcting Codes, Elsevier/North Holland, 1977. LINKS Nathan J. Russell, Table of n, a(n) for n = 1..49 G. Nebe, E. M. Rains and N. J. A. Sloane, Self-Dual Codes and Invariant Theory, Springer, Berlin, 2006. P. Gaborit, Tables of Self-Dual Codes E. M. Rains and N. J. A. Sloane, Self-dual codes, pp. 177-294 of Handbook of Coding Theory, Elsevier, 1998; (Abstract, pdf, ps). FORMULA From Nathan J. Russell, March 1, 2016: (Start) If 2n = 0 MOD 8 then a(n) = prod_(2^i+1, i=1,...,n-1) - prod_(2^i+1, i=0,...,n-2); If 2n != 0 MOD 8 then a(n) = prod_(2^i+1, i=1,...,n-1). If 2n = 0 MOD 8 then a(n) = A028362(n) - A028363( n/8); If 2n != 0 MOD 8 then a(n) = A028362(n). (End) MATHEMATICA From Nathan J. Russell, March 1, 2016: (Start) Table[ If[Mod[2 n, 8] == 0,   Product[2^i + 1, {i, 1, n - 1}] - Product[2^i + 1, {i, 0, n - 2}] ,   Product[2^i + 1, {i, 1, n - 1}]], {n, 1, 10}] (End) PROG (PYTHON) for n in range(1, 10):         product1 = 1         for i in range(1, n-1 + 1):             product1 *= (2**i+1)         if (2*n)%8 == 0:             product2 = 1             for i in range(0, n-2 + 1):                 product2 *= (2**i+1)             print str(product1 - product2)         else:             print str(product1) (PARI) a(n) = if (2*n%8==0, prod(i=1, n-1, 2^i+1)-prod(i=0, n-2, 2^i+1), prod(i=1, n-1, 2^i+1)) vector(20, n, a(n)) \\ Colin Barker, Feb 28 2016 CROSSREFS Cf. A003178, A003179, A028363, A028361. Sequence in context: A070826 A048599 A129731 * A088109 A231634 A128276 Adjacent sequences:  A269452 A269453 A269454 * A269456 A269457 A269458 KEYWORD nonn AUTHOR Nathan J. Russell, Feb 27 2016 EXTENSIONS a(20) corrected by Andrew Howroyd, Feb 22 2018 STATUS approved

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Last modified August 17 09:49 EDT 2018. Contains 313814 sequences. (Running on oeis4.)