# Interpolation Theory 101

Pre-requisites: There are a few assumptions made for the reader’s background. Mainly, that you’re familiar with the concept of a basis in the linear algebra sense and, in particular, function space bases (such as the monomials). Also, being familiar with turning a system of linear equations into it’s equivalent matrix form would be beneficial.

## Interpolation Problem Intro #︎

In interpolation, we wish to find a function $g(x)$ that satisfies $g(x_i) = y_i$ for some positive number $i$. We’ll call these constraints on the function $g(x)$. We say that the constraint is satisfied if $g(x)$ passes through the point $(x_i, y_i)$.

In the case of polynomial interpolation, we assume a form $$g(x) = \sum_{n=0}^N c_n \phi_n(x)$$

where $\phi_n(x)$ represents some set of basis functions. The simplest case of this is the monomials, where $\phi_n(x) = x^n$, such that:

$$g(x) = c_0 + c_1 x + c_2 x^2 + \dots + c_N x^{N}$$

This forms a basis for the space of $N$th order polynomials. However, we can choose $\phi_n$ to be any set of basis functions for polynomials, such as the Legendre, Laguerre, and Chebyshev polynomials (which all have their own special properties, not discussed here). You can even have non-polynomial basis functions, such as the Fourier series.

For just right now though, I’ll stick to talking about $g(x)$ living in the space of polynomials.

## Polynomial order #︎

A question does remain though: What kind (order) of polynomial should we try to interpolate with?

Well, if we have $m+1$ constraints, then we need, at minimum, a $m$th order polynomial to interpolate them. For example, we need 2 points ($m+1=2 \ \therefore m=1$) to make a line ($N = m = 1$ order polynomial).

So what about polynomials greater than order $m$? Well, then an infinite number polynomials satisfy the constraints. Example: If we only have 1 constraint ($m+1=1 \ \therefore m=0$) and there are an infinite number of lines that can satisfy that constraint.

We’d like for a solution to the problem to exist ($N \geq m$) and for that solution to be unique ($N < m+1$). Therefore, the order of the polynomial should be one less than the number of constraints ($N = m$).

Alternative Thought Process

An alternative way of thinking about it is that our constraints setup $m+1$ equations. To find a solution, we must have the number of unknowns match the number of equations. Since a $N$th order polynomial has $N+1$ unknowns, then $N + 1 = m + 1 \ \rightarrow \ N = m$.

## Solving the interpolation problem #︎

Now we have $m+1$ constraints, and know that $g(x)$ should be a $N = m$ order polynomial. So how do find $g(x)$?

The most straight forward way is to simply plug the constraints into $g(x)$. Doing this gives us the set of linear equations:

\begin{align*} g(x_0) &= c_0 + c_1 x_0 + c_2 x_0^2 + \dots + c_N x_0^{N} &= y_0 \\ g(x_1) &= c_0 + c_1 x_1 + c_2 x_1^2 + \dots + c_N x_1^{N} &= y_1 \\ \vdots \\ g(x_m) &= c_0 + c_1 x_m + c_2 x_m^2 + \dots + c_N x_m^{N} &= y_m \\ \end{align*}

We turn this into a matrix problem that looks like:

$$\begin{bmatrix} 1 & x_0 & x_0^2 & \dots & x_0^{N} \\[2pt] 1 & x_1 & x_1^2 & \dots & x_1^{N} \\[2pt] \vdots \\[2pt] 1 & x_m & x_m^2 & \dots & x_m^{N} \\[2pt] \end{bmatrix} \begin{bmatrix} c_0 \\ c_1 \\ \vdots \\ c_N \\ \end{bmatrix} = \begin{bmatrix} y_0 \\ y_1 \\ \vdots \\ y_m \\ \end{bmatrix}$$

The matrix above is known as a Vandermonde matrix. By solving this system, we find the coefficients $c_n$ that we can then reconstruct into $g(x) = \sum_{n=0}^N c_n x^n$.

## For arbitrary function basis #︎

Remember that we chose $\phi_n(x) = x^n$, which are the monomial bases. This works fine, but the resulting problem is very difficult to solve computationally (it is ill-conditioned). Other choices of $\phi_n(x)$ can help reduce the difficulty significantly, such as the aforementioned Chebyshev polynomials. In fact, we don’t even need $\phi_n(x)$ to be defined by a polynomial. For example, we could choose the Fourier series (so $g(x) = c_0 + \sum_{n=1}^N c_n \cos(nx) + s_n \sin(nx)$). This is discussed in more detail below.

Regardless of the choice of your basis functions, there’s one primary question: How do we go about finding the interpolating function using other bases?

Answer: We create a different matrix to solve with. Recall that we defined $g(x) = \sum_{n=0}^N c_n \phi_n(x)$. Using an arbitrary $\phi_n(x)$ instead of the monomials $x^n$, we can apply each constraint, expand our definition of $g(x)$, and get the following system of equations:

\begin{align*} g(x_0) &= c_0\phi_0(x_0) + c_1 \phi_1(x_0) + \dots + c_N \phi_N(x_0) &= y_0 \\ g(x_1) &= c_0\phi_0(x_1) + c_1 \phi_1(x_1) + \dots + c_N \phi_N(x_1) &= y_1 \\ \vdots \\ g(x_m) &= c_0\phi_0(x_m) + c_1 \phi_1(x_m) + \dots + c_N \phi_N(x_m) &= y_m \\ \end{align*}

This system of equation results in the matrix problem: $$\begin{bmatrix} \phi_0(x_0) & \phi_1(x_0) & \dots & \phi_N(x_0)\\ \phi_0(x_1) & \phi_1(x_1) & \dots & \phi_N(x_1)\\ \vdots \\ \phi_0(x_m) & \phi_1(x_m) & \dots & \phi_N(x_m)\\ \end{bmatrix} \begin{bmatrix} c_0 \\ c_1 \\ \vdots \\ c_N \\ \end{bmatrix} = \begin{bmatrix} y_0 \\ y_1 \\ \vdots \\ y_m \\ \end{bmatrix}$$

Just as before, we can then solve this matrix problem, then reconstruct $g(x)$ using the coefficients $c_n$.

## Slope constraints #︎

What if instead of wanting $g(x)$ to go through a specific point, we instead want the function to match a specific slope at a point? This is particularly useful for something like splines, where we want a smooth transition between one curve and another. Well, we can find a $g(x)$ that meets that constraint too!

First assume we replace the first interpolation constraint ($g(x_0) = y_0$) with a slope constraint ($g’(x_0) = s_0$), where $g’ = \partial g / \partial x$.

Recall that we set $g(x) = \sum_{n=0}^N c_n \phi_n(x)$. To find $g’(x)$, we can simply take the derivative of sum expression:

\begin{align*} \frac{\partial}{\partial x} g(x) &= \frac{\partial}{\partial x} \left [c_0\phi_0(x) + c_1 \phi_1(x) + \dots + c_N \phi_N(x) \right] \\ &= \frac{\partial}{\partial x} c_0\phi_0(x) + \frac{\partial}{\partial x} c_1 \phi_1(x) + \dots + \frac{\partial}{\partial x} c_N \phi_N(x) \\[12pt] g'(x) &= c_0\phi'_0(x) + c_1 \phi'_1(x) + \dots + c_N \phi'_N(x) \end{align*}

So the derivative of $g(x)$ is just the sum of the derivatives of $\phi_n(x)$ weighted by the coefficients $c_n$. This is quite nice, as our function coefficients for $g(x)$ are the same as for $g’(x)$.

Using this result, we can write our slope constraint as:

$$g’(x_0) = c_0\phi_0’(x_0) + c_1 \phi_1’(x_0) + \dots + c_N \phi_N’(x_0) = s_0$$

Recall that we replace our first interpolation constraint $g(x_0) = y_0$ with the slope constraint $g’(x_0) = s_0$. We can then replace the first equation in the linear system described above with $g’(x_0) = s_0$. This results in the matrix problem: $$\begin{bmatrix} \phi_0'(x_0) & \phi_1'(x_0) & \dots & \phi_N'(x_0)\\ \phi_0(x_1) & \phi_1(x_1) & \dots & \phi_N(x_1)\\ \vdots \\ \phi_0(x_m) & \phi_1(x_m) & \dots & \phi_N(x_m)\\ \end{bmatrix} \begin{bmatrix} c_0 \\ c_1 \\ \vdots \\ c_N \\ \end{bmatrix} = \begin{bmatrix} s_0 \\ y_1 \\ \vdots \\ y_m \\ \end{bmatrix}$$

This looks nearly identical to our pure-interpolation matrix problem! The only difference is that the first row of our matrix replaces $\phi_n \rightarrow \phi_n’$ and the first entry on the right hand side vector replaces $y_0 \rightarrow s_0$.

This last result is incredibly powerful. Using it, we can actually solve differential equations. This will be discussed in a later article.

## Appendix A: Further thoughts on non-polynomial functions #︎

When using non-polynomials basis functions, terminology can change a bit, but the concepts are the same.

### Required Degrees-of-freedom for $g(x)$ #︎

For example, we stated that for $m+1$ constraints, we need an $m$th order polynomial. For non-polynomial basis function, we instead say that we need the number of degrees-of-freedom (DOFs) $N+1$ to equal the number of constraints $m+1$. For basis functions, the DOFs are the coefficients. Note this rule still applies to polynomials (a $N$th order polynomial has $N+1$ coefficients), but saying the order of the polynomial is more common-place and can be simpler to relate to the polynomial itself.

Note that it may seem confusing to have both number of DOFs and number of constraints denoted as $X+1$ rather than simply $X$. This is due to the fact that conventionally the zero index for DOFs corresponds to the constant function, which is the 0th order polynomial. This convention often extends to non-polynomial functions as well (ie. Fourier series). We apply the same logic to the constraints to so that we can make simple statements like $N = m$ (which inherently means that $N+1 = m+1$).

### Non-linear $g(x)$ with respect to degrees-of-freedom #︎

Also, note that we’ve restricted ourselves to function bases. This means that we can express $g(x)$ as a sum of those function bases times coefficients, which allows us to separate the coefficients away from the basis functions. However, we can work with expressions of $g(x)$ that are non-linear with respect to their degrees-of-freedom. An example of that is an alternate form of the Fourier series:

$$g(x) = \frac{c_0}{2} + \sum_{n=1}^N c_n \cos(2\pi n x + \psi_n)$$

Note that $\psi_n$ is a degrees-of-freedom for $g(x)$, but we can’t take it out of the sum; it’s inside the $\cos$ expression. This must be solved as a non-linear equation, which can’t be expressed as a solution to a matrix problem. In order to solve this, you must express the problem in a residual form, and use a non-linear solver to find the solution. I’ll quickly cover the former, but not the latter.

First, I’ll denote $g(x)$ as $g(\mathbf c; x)$, to show that $g$ is a function of the $x$ function evaluation, but also the set of DOFs $\mathbf c$. To create the residual form, we form a system of equations of the form:

\begin{align*} g(\mathbf c; x_0) - y_0 &= 0 \\ g(\mathbf c; x_1) - y_1 &= 0 \\ \vdots \\ g(\mathbf c; x_m) - y_m &= 0 \\ \end{align*}

Using this, we get the vector-valued residual function for this system:

$$R\left(g(\mathbf c; x)\right) = R \left( \begin{bmatrix} c_0 \\ c_1 \\ \vdots \\ c_N \\ \end{bmatrix} \right) = \begin{bmatrix} g(\mathbf c; x_0) - y_0\\ g(\mathbf c; x_1) - y_1\\ \vdots \\ g(\mathbf c; x_m) - y_m\\ \end{bmatrix}$$

The non-linear solve then finds the set of degrees-of-freedom such that $R(g(\mathbf c; x)) = \mathbf 0$.