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 A068620 Number of strings over Z_4 of length n with trace 0 and subtrace 0. 10
 1, 2, 4, 8, 56, 272, 1184, 4736, 17536, 65792, 254464, 1015808, 4130816, 16781312, 67641344, 270565376, 1077968896, 4295032832, 17146445824, 68585259008, 274609995776, 1099512676352, 4400196091904, 17600784367616, 70385932435456, 281474993487872 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n; 0,0) 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 F. Ruskey, Strings over Z_4 of given Trace and Subtrace 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.: -x*(704*x^7-704*x^6+288*x^5-56*x^4+32*x^3-24*x^2+8*x-1) / ((2*x-1)*(4*x-1)*(8*x^2-4*x+1)*(16*x^4+1)). - Colin Barker, Dec 06 2014 EXAMPLE a(3;0,0)=4 since the four 4-ary strings of trace 0, subtrace 0 and length 3 are { 000, 022, 202, 220 }. CROSSREFS Cf. A068711, A068774, A068777, A068786, A068778, A068787, A068788, A068789, A068790. Sequence in context: A058816 A018446 A289324 * A073953 A206850 A094333 Adjacent sequences:  A068617 A068618 A068619 * A068621 A068622 A068623 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 19 2007 Terms a(11) onward from Max Alekseyev, Apr 14 2013 STATUS approved

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Last modified November 19 05:29 EST 2018. Contains 317333 sequences. (Running on oeis4.)