Define coupled recurrences un=un−1+vn−1u_n = u_{n-1} + v_{n-1}un=un−1+vn−1 and vn=un−1v_n = u_{n-1}vn=un−1 with u0=1u_0 = 1u0=1, v0=0v_0 = 0v0=0. Find u4u_4u4.
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