0,5

G.f. : (1-2x+x^2)/(1-3x+3x^2-x^3-3x^4); a(n)=3a(n-1)-3a(n-2)+a(n-3)+3a(n-4); a(n)=sum{k=0..floor(n/3), binomial(n-k, 3k)3^k}.

Sequence in context: A106302 A158842 A100136 this_sequence A098536 A011937 A097122

Adjacent sequences: A097117 A097118 A097119 this_sequence A097121 A097122 A097123

easy,nonn

Paul Barry (pbarry(AT)wit.ie), Jul 25 2004