I’m trying to solve a system of differential equations given in Cordier et al., 4702 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 4, DECEMBER 1999.

The equations are quite complicated, so I won’t try to replicate them here. In the simplified version below there are two fields fields Ay and Ap, along with their associated test functions Wy, Wp.

```
fespace FE(Th, P1);
FE Ay, Ap, Wy, Wp;
problem Cordier(Ay, Wy, Ap, Wp) = int2d(Th)
(
Ap*Wp
+ dx(Ay)*dx(Wy)
+ Ap*dx(Wp)
+ Wp*dx(Ap)
)
-int2d(Th)(Wy);
```

The code gives the following error message at the line Ap*Wp:

"+ error operator * <10LinearCombI6MDroit4C_F0E>, <10LinearCombI6MDroit4C_F0E> "

Any ideas as to why I’m getting this error? Are certain combinations of Ay, Ap, Wy, and Wp not allowed?