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 A068774 Number of strings over Z_4 of length n with trace 0 and subtrace 2. 11
 0, 0, 0, 8, 40, 240, 1120, 4480, 17280, 65280, 253440, 1015808, 4126720, 16773120, 67624960, 270499840, 1077903360, 4294901760, 17146183680, 68585259008, 274608947200, 1099510579200, 4400191897600, 17600767590400, 70385915658240, 281474959933440 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS a(n; 0,2) where a(n; t,s) is the number of length n 4-ary strings whose digits sum to t mod 4 and whose sum of products of all pairs of digits sum to s mod 4. LINKS Max Alekseyev, PARI/GP scripts for miscellaneous math problems FORMULA a(n; t, s) = a(n-1; t, s) + a(n-1; t+3, s+3t+1) + a(n-1; t+2, s+2t) + a(n-1; t+1, s+t+1) where t is the trace and s is the subtrace. Empirical g.f.: 8*x^4*(40*x^4-40*x^3+20*x^2-5*x+1) / ((2*x-1)*(4*x-1)*(8*x^2-4*x+1)*(16*x^4+1)). - Colin Barker, Dec 06 2014 CROSSREFS Cf. A068620, A068711, A068777, A068786, A068778, A068787, A068788, A068789, A068790. Sequence in context: A228679 A188234 A068775 * A073955 A264332 A188332 Adjacent sequences:  A068771 A068772 A068773 * A068775 A068776 A068777 KEYWORD easy,nonn AUTHOR Frank Ruskey and Nate Kube, Aug 15 2002 EXTENSIONS Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, May 20 2007 Terms a(11) onward from Max Alekseyev, Apr 09 2013 STATUS approved

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Last modified July 24 20:26 EDT 2021. Contains 346273 sequences. (Running on oeis4.)