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%I #5 Mar 30 2012 18:39:21
%S 1,1,1,1,-1,-2,0,-10,29,-689,79779,-306741785,-537059115735445,
%T 32782820784889194174016393,
%U 123899509182081496572342154385115870319926534,6620488215208352146569523585853411114659592927465468100410366988520167986216
%N Imaginary part of Propp's cubic recurrence starting with i (i^2=-1).
%C b(n)=x(n)+i*y(n) where x(n) and y(n) are integer sequences.
%F a(n)=Im(b(n)) where b(1)=b(2)=b(3)=b(4)=i and b(n)=(1+b(n-1)*b(n-2)*b(n-3))/b(n-4)
%Y Cf. A051786, A086889.
%K sign
%O 1,6
%A _Benoit Cloitre_, Sep 17 2003