# how can i define a matrix product like a summation

first, my mother tongue is not English but I try it, my problem is that if I define a symbolic matrix V such that,

var('V_11,V_12,V_13,V_21,V_22,V_23,V_31,V_32,V_33')

```
V = matrix(SR,3,3,[V_11,V_12,V_13,V_21,V_22,V_23,V_31,V_32,V_33])
```

and then I define

```
P = [V[a,k]*V[b,k]*V[a,j]*V[b,j] for k in [0,1,2] for j in [0,1,2]]
```

But if I evaluate, for example

```
P(a=0,b=1)
```

but the code is not work can someone help me?