# Can polynomial regression be used to handle data with more than one dimension?

Polynomial regression is said to be a subclass of linear regression. However, it seems that only linear regression can handle data input with more than one dimension, whereas polynomial regression cannot. Is my understanding correct?

For example,

Given a set of $$N$$ data pairs $$\{(x_k,t_k)\}_{k = 1}^N, x_k \in \mathbb{R}^n, t_k\in \mathbb{R}$$

A linear regression uses the model

$$y(x,t) = \sum\limits_{i = 1}^n w_i x_i$$ where $$w_i$$ are the coefficients of my model

Then the empirical loss is written as

$$E = \|Xw - t\|$$^2

where $$X = \begin{bmatrix} 1 & x_1^T \\ \vdots & \vdots \\ 1 & x_N^T \end{bmatrix}$$, $$w = \begin{bmatrix} 1 & w_1 & \ldots & w_n\end{bmatrix}^T$$

In the Polynomial regression case, my model is,

$$y(x,t) =\sum\limits_{i = 0}^n w_ix^i = w_0 + w_1x + w_2x^2 + \ldots w_nx^n$$

Here, $$x$$ no longer represents the entries of a vector $$x \in \mathbb{R}^n$$. But just a single point $$x \in \mathbb{R}$$.

I think the issue here is that powers are not defined for non-1D vectors.

• Powers are defined: they are called "tensor products." – whuber Jan 25 '19 at 13:32

Here is an example 3D surface quadratic polynomial and data.

• How would you formulate this problem in terms of the loss? – Carlos - the Mongoose - Danger Jan 25 '19 at 14:28
• instead of "y=f(x)", I used "z=f(x,y)" and standard lowest sum-of-squared-error linear regression. For example, a flat surface would be "z = a + bx + cy". – James Phillips Jan 25 '19 at 16:27

I will quote wikipedia because it is really well said.

• But Wikipedia doesn't contradict what I said. Look, each row of the matrix $X$ looks like $[1, x_1, x_1^2, \ldots, x_1^m]$. But in linear regression, each row of the matrix $X$ consists of all the elements of the vector $[1, x_{11}, x_{12}, \ldots, x_{1m}]$. These are just totally different objects – Carlos - the Mongoose - Danger Jan 25 '19 at 14:27