= Linear precision at Greville abscissae
{title2=$\sum_i\xi_iN_{i,d}(t)=t$}
For an open clamped <spline knot sequence> and degree $d\ge1$, the <Greville abscissae> $\xi_i=d^{-1}\sum_{r=1}^dt_{i+r}$ reproduce the parameter. The <B-spline differentiation formula> gives derivative coefficients $d(\xi_i-\xi_{i-1})/(t_{i+d}-t_i)=1$. Their lower-degree <partition of unity> makes the derivative of the weighted sum equal to one. The left endpoint value is the left parameter endpoint, fixing the integration constant to zero. This proves <linear precision> even with unequal knot intervals.
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