# Problem with syntax error in writing problem

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?

You need to use a pair of functions.

``````fespace FE(Th, [P1, P1]);
FE [Ay, Ap], [Wy, Wp];
problem Cordier([Ay, Ap], [Wy, Wp]) = ...
``````

the answer of Pierre is Ok but the space pair of function is not mandatory

the the order in problem is list on unwon function , list of test function

So you have to write :

``````fespace FE(Th, P1);
FE Ay, Ap, Wy, Wp;

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