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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
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.
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 }.
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