1 | initial version |

No need to include `psi`

in your initial function declaration,
as you then define `psi`

in the next line.

Not sure how to explain the error you get, but here is something that works:

```
sage: psi1, psi2 = function('psi_1 psi_2')
sage: psi(t) = 1/sqrt(2) * (psi_1(t) + psi_2(t) * i)
```

This defines `psi`

as a function:

```
sage: psi
t |--> 1/2*sqrt(2)*(psi_1(t) + I*psi_2(t))
```

and you can differentiate it:

```
sage: diff(psi(t), t)
1/2*sqrt(2)*(diff(psi_1(t), t) + I*diff(psi_2(t), t))
```

2 | No.2 Revision |

No need to include `psi`

in your initial function declaration,
as you then define `psi`

in the next line.

Not sure how to explain the error you get, but here is something that works:

`sage: psi1, psi2 = `~~function('psi_1 ~~function('psi_1, psi_2')
sage: psi(t) = 1/sqrt(2) * (psi_1(t) + psi_2(t) * i)

~~This ~~That defines `psi`

as a ~~function:~~function. Observe the result:

```
sage: psi
t |--> 1/2*sqrt(2)*(psi_1(t) + I*psi_2(t))
```

~~and you ~~That function can ~~differentiate it:~~be differentiated:

```
sage: diff(psi)
t |--> 1/2*sqrt(2)*(diff(psi_1(t), t) + I*diff(psi_2(t), t))
sage: diff(psi(t), t)
1/2*sqrt(2)*(diff(psi_1(t), t) + I*diff(psi_2(t), t))
```

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