# Second order dxx, solver crash **URL:** \ **Category:** General Discussion\ **Created:** [March 30, 2022, 3:18pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635 "2022-03-30T15:18:42Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![tess35](https://avatars.discourse-cdn.com/v4/letter/t/ac8455/32.png) [@tess35](https://community.freefem.org/u/tess35)\ **Post date:** [March 30, 2022, 3:18pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635/1 "2022-03-30T15:18:42Z")
``` Hi everybody, I have a difficulty with the second order dxx(). ``` Take a simple model, u’’’(x)=1 \<=\> int2d u’’(x)_v(x)=int2d 6_v(x). The solution is polynomial of degree 3, like x^3 + constant mesh Th=square(100,100); fespace Vh(Th, P2); Vh u, v; solve equa(u,v,solver=UMFPACK) = int2d(Th) ( -dxx(u)_dx(v) ) - int2d(Th) ( 6.0_v ) + on( 4, u= 1.0 )+ on( 2, u= 2.0 ) ; But it doesn’'t work. The example bilapMorley.edp for the bilaplacien work perfectly. I modify it to have a 1D problem but the solver crash. Is it a problem of formulation or the fespace is not correct ? ``` Sincerely ``` ---
**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:** [March 31, 2022, 4:31pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635/2 "2022-03-31T16:31:21Z")
First, You use 2D finite element for 1d problem, secondly, you use P2 finite element to compute third derivative third: you vationnal form is crazy if v = 1 then you have 0 = 6 =\> system not invertible . A code in 1d (wrong ) the formulation is wrong pb of Boundary condition you have 3 constants unknown and you set only 2 … ```auto load "msh3" meshL Th=segment(100); fespace Vh(Th, P2); Vh u, v ,xx=x, xxx=x*x*x; solve equa(u,v,solver=UMFPACK) = int1d(Th) ( -dxx(u)*dx(v) + 1e-5*dx(u)*dx(v)) - int1d(Th) ( 6*(v) ) + on(1,2,u=xxx); plot([xx[],u[]],[xx[],xxx[]],wait=1); ``` ---
**Author:** ![tess35](https://avatars.discourse-cdn.com/v4/letter/t/ac8455/32.png) [@tess35](https://community.freefem.org/u/tess35)\ **Post date:** [April 13, 2022, 10:27pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635/3 "2022-04-13T22:27:22Z")
``` Hi, ``` I have some setbacks. Your code does’nt work on my machine, is it abnormal or voluntary ? The error code is : > Build Nodes/DF on mesh : n.v. 101, n. elmt. 100, n b. elmt. 2 > nb of Nodes 201 nb of DoF 201 DFon=1100 > – FESpace: Nb of Nodes 201 Nb of DoF 201 > current line = 11 > Assertion fail : (0) > line :245, in file …/femlib/P012\_3dCurve.cpp > Assertion fail : (0) > line :245, in file …/femlib/P012\_3dCurve.cpp > err code 6 , mpirank 0 I have ever tried to reinstall 2 times from the binaries, version 4.6 and to compile from the source , version 4.10. Before that, i used freefem 4.4 which don’t use 1 D element, it’s why my code use 2 D element. I’m much more used to finite difference method , sorry for my dumb question. I don’t understand why P2 finite element is bad in this case ? Because I need dxx so second order ? 3 constants is needed, now i see that, x³+1 is not the only solution without enough constraints. About the third remark, did you want to say that we have a conflict between int1d(Th)(6\*v) and on(2,u=xxx ) (xxx=0) if v=1 at x=0 ? But applying the integration by parts : y’’ =6 =\> y’‘\*v =6 \*v \<=\> int2d -y’‘\*v’ + int1d y’‘_v = int2d 6_v \<=\> int2d y’\*v’’ - int1d y’\*v’ + int1d y’'_v = int2d 6_v I suppose that a factor is missing ? I tried several patchs but nothing work. ``` Thank for your reply ``` ---
**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:** [April 15, 2022, 12:18pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635/4 "2022-04-15T12:18:36Z")
You have to use the last FreeFem version 4.11 no version 4.6 (Thu Apr 2 14:11:06 2020 +0200) ---
**Author:** ![tess35](https://avatars.discourse-cdn.com/v4/letter/t/ac8455/32.png) [@tess35](https://community.freefem.org/u/tess35)\ **Post date:** [April 19, 2022, 8:46pm UTC](https://community.freefem.org/t/second-order-dxx-solver-crash/1635/5 "2022-04-19T20:46:13Z")
Now it work with the version 4.11 from binary packages, (the last test was under 4.10 ). I tried to add : +int0d(Th,1) (dxx(u)\*v) for dxx(u)=0 at x=0, or +int0d(Th,2) (dxx(u)_v) - int0d(Th,2) ( 6_v ) for dxx(u)=6 at x=1 but nothing work.