# Confusion regarding the notation of associated operator of a PDE.

I'm given the following PDE: $$u_{tt}-u_{xx}+u^3=0$$ My source says that the associated operator is $$L:=\frac{\partial^2}{\partial t^2}-\frac{\partial^2}{\partial x^2}+u^2$$ which is arrived at simply by factoring the common term $$u$$ and thinking of the operator as left multiplying the function.$$\frac{\partial^2}{\partial t^2}u-\frac{\partial^2}{\partial x^2}u+u^3=\left (\frac{\partial^2}{\partial t^2}-\frac{\partial^2}{\partial x^2}+u^2 \right )u=Lu=0$$ Is this correct? In general how do I find and write the associated operator of a PDE? Do I simply do as my source does and just factor out the $$u$$? For eg. is the operator in $$Lu=u_x+u_y+1$$ given by $$L= \frac{\partial}{\partial x}+\frac{\partial}{\partial y}+\frac{1}{u}$$ as given here?

• It's not linear because of the $u^3$ term Mar 14, 2019 at 7:24
• @Dylan My source of confusion is how to write the operator. The linearity part I've understood and since it wasn't relevant, I've removed it from the question.
– ZSMJ
Mar 14, 2019 at 7:32
• The source you cite uses rather ambiguous notation. You cannot simply "factor" the term $\partial^2_t - \partial^2_x + u^2$, as it will still depend on the function $u$. The "operator" should be an object which is a priori independent of the function: it should be an object which takes a function $u$ and maps it to a new function $f$.
– char
Mar 26, 2019 at 21:21
• Remaining in your context. Perhaps it is better to consider the "operator" $Lu = (\partial^2_t - \partial^2_x) u + f(u)$, where $f(x) = x^3$ for $x \in \mathbb{R}$. This allows you to see why exactly the resulting equation is nonlinear, and how you may proceed in linearizing it (as suggested in the previous comments).
– char
Mar 26, 2019 at 21:23
• @bgsk Yes, breaking it down like this makes sense and makes it easy to see the non linearity. I was really confused by how he just factored out the $u$. Could you paste these comments as an answer so I can give you the bounty? Also, could you have a look at this and recommend some books?
– ZSMJ
Mar 27, 2019 at 4:46

As indicated by the OP, I will regroup my comments as an answer.

The source notes you cite make use of rather ambiguous/confusing notation. Indeed, one cannot simply "factor out" the term $$\partial_t^2 - \partial_x^2 + u^2$$ and call it "the operator", as this object clearly depends on the function $$u$$. The operator should be an object which is a priori independent of the function: it takes a "function" $$u$$ as input and yields another "function" $$g$$ as output.

In relation to your actual question. I believe it is better to consider the "operator" $$Nu = (\partial_t^2 - \partial_x^2)u + f(u)$$, where $$f(x) = x^3$$ for $$x \in \mathbb{R}$$. It is now clear that the operator $$N$$ is nonlinear, and the linear and nonlinear parts are moreover explicitly split. For futher reference, the linear operator $$L = \partial_t^2 - \partial_x^2$$ is called the wave operator (also called the d'Alembertian), as it is the governing linear operator in the wave equation $$$$\partial_t^2 u - \partial_t^2 u = 0.$$$$ Hence the equation associated to the "operator" you were considering is the nonlinear wave equation $$$$\partial_t^2 u - \partial_t^2 u + u^3 = 0.$$$$