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A068786
S(n; 1,0) = S(n; 3,0) where S(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.
9
1, 2, 6, 16, 40, 192, 896, 4096, 17536, 69632, 270336, 1048576, 4126720, 16515072, 66584576, 268435456, 1077968896, 4311744512, 17213423616, 68719476736, 274608947200, 1098437885952, 4395899027456, 17592186044416, 70385932435456, 281543696187392
OFFSET
1,2
FORMULA
S(n; t, s) = S(n-1; t, s) + S(n-1; t+3, s+3t+1) + S(n-1; t+2, s+2t) + S(n-1; t+1, s+t+1).
Empirical g.f.: x*(1-6*x+14*x^2-16*x^3+8*x^4-32*x^5+32*x^6) / ((1-4*x)*(1-4*x+8*x^2)*(1+16*x^4)). - Colin Barker, Feb 21 2016
KEYWORD
nonn,base
AUTHOR
Frank Ruskey, Mar 29 2002
EXTENSIONS
Terms a(11) onward from Max Alekseyev, Apr 09 2013
STATUS
approved