B-splines and periodic or natural boundaries

Hi again,
after having got going with B-splines in >1D thanks to Peter's help, I
am now wondering about implementing different boundary conditions for
interpolation; I ended up using the routines from pppack, but it seems
that they use mostly not-a-knot boundaries or some other stuff, but
never periodic or natural boundaries. I am not sure if the latter can
be made with B-splines, but the periodic ones should be possible.
If k is the rank of the B-spline, there are k extra knots to be
considered at either end of the interval. In the examples from de
Boor's book with the not-a-knot b.c., these knots are all the same.
What I did so far was to continue the knot sequence periodically at
both ends, but oddly (for me), the result is just the same as in the
original example. Can somebody tell me if the pppack routines can be
used straightforwardly with other b.c.s and what it takes to do that?
Thanks,
Tom

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