1(b)

The global error at x_n is the sum of the differences $\psi_{j-1}(x_n) - \psi_j(x_n)$, $j = 1 \ldots n$, which might be called the effects at x = x_n of the local errors $\psi_{j-1}(x_j) - y_j$ at each step. But these effects may shrink or grow.