Hello everybody,

I am trying to solve a single truss element problem defined by two points A and B. In each point I have two DoF i.e. u1,u2,u3,u4.

I tried to use the tutorial for 2d beams see and an old thread `but I could not solve my problem defining the correct mesh and boundary condition.

So far I have this code:

```
import numpy as np
from dolfin import *
# Material variables
E = Constant(1e5)
nu = Constant(0.3)
mu = E/2/(1+nu)
lmbda = E*nu/(1+nu)/(1-2*nu)
# Define Single Truss mesh from point A to point B with DoF 2 in each point
#
#
mesh = Mesh(mesh)
V = VectorFunctionSpace(mesh, 'Lagrange', degree=1)
# Define Dirichlet boundary i.e. left point A is fixed
#
#
def eps(v):
return sym(grad(v))
def sigma(u):
d = u.geometric_dimension()
I = Identity(d)
return lmbda * tr(eps(u)) * I + 2 * mu * eps(u)
# Solution variables
dx = Measure("dx")
du = TrialFunction(V)
U = Function(V)
u = U.vector()
u_ = TestFunction(V)
d = U.geometric_dimension()
I = Identity(d)
f = Constant((0, 0))
# Assembly system
a = inner(sigma(du), eps(u_)) * dx
L = inner(f, u_) * dx
K, f_vec = assemble_system(a, L, bc)
# External force on point B
f_vec[3] = 0
f_vec[4] = -1
# Solve system
solve(K, u, f_vec)
```