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I have some questions about the definition of splines and in particular periodic splines. So in non periodic case splines are piecewise polynomials of degree $<=m$ which are $m-1$ times differentiable.(I took $m-1$ time differentiability for simplicity) By this definition we can call linear or quadratic polynomials splines of degree 3.

For periodic case we need boundary conditions on some interval$(a,b)$ $s^{(q)}(a)=s^{(q)}(b),q=1...m-1$. Does it mean that periodic splines are piecewise polynomials of degree $m$(not smaller then $m$) or constants?

Is there analogue for truncated polynomials basis in periodic case?

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    @Katja: Right, I figured the "which satisfies the boundary conditions for degree-$m$ splines" was implied. I think now your question is clear.2011-01-21

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Wlog we can take the knot at $x=0$, and then we're looking for two polynomials of degree $\le m$ which join there with $C(m-1)$ continuity.

If the two polynomials both have degree $m-1$ then, since they have $m$ derivatives (including the 0th derivative) in common they are equal. (Whether you consider a polynomial to be a spline on the basis that it's piecewise polynomial with all the pieces having the same function is up to you).

However, it's possible for one to be of degree $m$ and the other of degree $m-1$. Consider the case $m=2$. Our polynomials are $f(x) = ax^2 + bx + c$ and $g(x) = dx + e$. $f(0) = g(0) \Rightarrow c = e$, $f^\prime(0) = g^\prime(0) \Rightarrow b = d$. $a$ is unconstrained.

Example

$f(x) = \left\{\begin{array}{l} (x-1)^2 & 0 <= x < 2 \\ 2x - 3 & 2 <= x < 3 \\ 4 - (x-4)^2 & 3 <= x < 5 \\ 13 - 2x & 5 <= x < 6 \\ f(x-6) & x >= 6 \\ f(x+6) & x < 0 \end{array}\right.$

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    @Katja, added example2011-01-27