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The Greville abscissae are defined to be the mean location of *k-1*
consecutive knots in the knot vector for each basis spline function of order
*k*. With the first and last knots in the `gsl_bspline_workspace`

knot vector excluded, there are `gsl_bspline_ncoeffs`

Greville abscissae
for any given B-spline basis. These values are often used in B-spline
collocation applications and may also be called Marsden-Schoenberg points.

- Function:
*double***gsl_bspline_greville_abscissa***(size_t*`i`, gsl_bspline_workspace *`w`); Returns the location of the

*i*-th Greville abscissa for the given B-spline basis. For the ill-defined case when*k=1*, the implementation chooses to return breakpoint interval midpoints.