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A068620
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Number of strings over Z_4 of length n with trace 0 and subtrace 0.
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10
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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
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OFFSET
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1,2
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COMMENTS
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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.
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LINKS
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FORMULA
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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
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EXAMPLE
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a(3;0,0)=4 since the four 4-ary strings of trace 0, subtrace 0 and length 3 are { 000, 022, 202, 220 }.
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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