# Doubly indexed sum

I would like to define a doubly indexed sum. Below is what I did:

```
V=list(var(','.join(['a_%d%d' % (i,n) for i in [0..5] for n in [0..5]])))
sum(a_in*binomial(5,i)*binomial(5,n) for i in [0..5] for n in [0..5])
```

I get the global name 'a_' is not defined error. I tried this when there is only one index and it works, but doesnt seem to work for double index. Can anyone help me with this?