# TypeError 'unable to simplify to float approximation' while trying to define an integral operator

Hello, I am trying to define an integral operator acting on a subspace of $L^2(\mathbb{R})$ that depends of time using sage. Being more explicit $W(t)$ takes a real function $f$ and returns another function $g$ defined by \begin{equation} W(t)f(x)=g(x)=\int_{-\infty}^{+\infty}K(x,y,z)f(y)\text{d}y \end{equation} This is the code I came up with

```
def W(t):
def dummy2(f):
def dummy3(x):
integrand(y)=f(y)*K(x,y,t)
return numerical_integral(integrand,-Infinity,+Infinity,algorithm='qag')[0]
return dummy3
return dummy2
```

With $K$ a reasonable function of $x,y,t$ However I am getting the following error

```
TypeError: unable to simplify to float approximation
```

I am pretty sure it's related to the types passed to numerical_integrand What could be a solution? Or a better way to implement it? Thank you all

The code looks fine. (Maybe change to

`integrand = lambda y: f(y)*K(x,y,t)`

). What is the function $K$ and the call that yields the error? Note $K$ must return a constant which has a floating point approximation, and hence cannot contain symbolic variables (though you could work around this by considering them as formal variables).I still get an error, I defined K as

How can I define them as formal variables?