1 | initial version |

The way you define `dn`

is a *symbolic function*, that is an expression given by a mathematical formula, not something that produces an object.

For `plot_dn`

if you want to produce a graphical object, you need to define is a Python function, not a symbolic. You can try the following:

```
sage: plot_dn = lambda n, theta, y : plot(dn(k, n, theta, y), (k, 0, 1))
```

It is equivalent to:

```
sage: def plot_dn(n, theta, y):
....: return plot(dn(k, n, theta, y), (k, 0, 1))
```

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