# Problem with syntax error in writing problem

**URL:** <https://community.freefem.org/t/problem-with-syntax-error-in-writing-problem/677>\
**Category:** General Discussion\
**Created:** [December 9, 2020, 3:34am UTC](https://community.freefem.org/t/problem-with-syntax-error-in-writing-problem/677 "2020-12-09T03:34:20Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![amtravco](https://avatars.discourse-cdn.com/v4/letter/a/13edae/32.png) [@amtravco](https://community.freefem.org/u/amtravco)\
**Post date:** [December 9, 2020, 3:34am UTC](https://community.freefem.org/t/problem-with-syntax-error-in-writing-problem/677/1 "2020-12-09T03:34:21Z")

</div>

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?

---

<div class="post-metadata">

**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\
**Post date:** [December 9, 2020, 6:23am UTC](https://community.freefem.org/t/problem-with-syntax-error-in-writing-problem/677/2 "2020-12-09T06:23:44Z")

</div>

You need to use a pair of functions.

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

```

---

<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [December 9, 2020, 4:37pm UTC](https://community.freefem.org/t/problem-with-syntax-error-in-writing-problem/677/3 "2020-12-09T16:37:56Z")

</div>

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 :

```auto
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);

```