Suppose that \(x\) satisfies the equation \[x^3=2x+1.\]

  1. For every integer \(k\geq0\), we can uniquely write \[x^k=A_k+B_kx+C_kx^2\] where \(A_k\), \(B_k\), \(C_k\) are integers. So, in part (i), it was shown that \[A_4=0,\ B_4=1,\ C_4=2 \qquad\text{and}\qquad A_5=2,\ B_5=4,\ C_5=1.\] Show that \[A_{k+1}=C_k,\qquad B_{k+1}=A_k+2C_k,\qquad C_{k+1}=B_k.\]

What happens if we multiply \(x^k=A_k+B_kx+C_kx^2\) by \(x\)?

  1. Let \[D_k=A_k+C_k-B_k.\] Show that \(D_{k+1}=-D_k\) and hence that \[A_k+C_k=B_k+(-1)^k.\]

If \(D_{k+1}=-D_k\), then can we come up a formula for \(D_k\)?