%I A050228
%S A050228 1,3,6,11,19,31,49,76,116,175,262,390,578,854,1259,1853,2724,4001,5873,
%T A050228 8617,12639,18534,27174,39837,58396,85596,125460,183884,269509,394999,
%U A050228 578914,848455,1243487,1822435,2670925,3914448,5736920,8407883
%N A050228 a(n) is the number of subsequences {s(k)} of {1,2,3,...n} such that s(k+1)-s(k) 
               is 1 or 3.
%C A050228 The second differences c(n) of {a(n)} satisfy c(n)=c(n-1)+c(n-3) and 
               give A000930 with the first 5 terms deleted.
%C A050228 Partial sums of A077868. - Paul Barry (pbarry(AT)wit.ie), Sep 16 2004
%F A050228 G.f. : x/((1-x)^3-x^3(1-x)^2); a(n)=3a(n-1)-3a(n-2)+2a(n-3)-2a(n-4)+a(n-5); 
               a(n-1)=sum{k=0..floor(n/3), binomial(n-2k, k+2)}. - Paul Barry (pbarry(AT)wit.ie), 
               Sep 16 2004
%p A050228 with(combstruct): SubSetSeqU := [T, {T=Subst(U,U), S=Set(U, card>=3), 
               U=Sequence(Z, card>=3)}, unlabeled]: seq(count(SubSetSeqU, size=n), 
               n=9..46); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 18 
               2008
%Y A050228 Sequence in context: A091094 A116100 A004133 this_sequence A114089 A001976 
               A144115
%Y A050228 Adjacent sequences: A050225 A050226 A050227 this_sequence A050229 A050230 
               A050231
%K A050228 nonn
%O A050228 1,2
%A A050228 John W. Layman (layman(AT)math.vt.edu), Dec 20 1999