# unable to simplify to float approximation with a gamma function

I'm very new to Sage, so please forgive me if this is a trivial question. I've been trying to specify a function as follows:

```
p(x,a,b) = x * math.gamma(a + b)
```

However, I get the following error message:

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

How do I specify the function p() that contains a Gamma function? I was thinking I could create a Python function (i.e. starting with def(x,a,b)) and then put the code inside the function body, but I have a feeling I'm missing something.