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# The first step is to define the variational problem at hand. We define
# the variational problem in UFL terms in a separate form file
# {download}`demo_poisson/poisson.py`. We begin by defining the finite
# element:
from basix.ufl import element
from ufl import (
Coefficient,
Constant,
FunctionSpace,
Mesh,
TestFunction,
TrialFunction,
ds,
dx,
grad,
inner,
)
e = element("Lagrange", "triangle", 1)
# The first argument to :py:class:`FiniteElement` is the finite element
# family, the second argument specifies the domain, while the third
# argument specifies the polynomial degree. Thus, in this case, our
# element `element` consists of first-order, continuous Lagrange basis
# functions on triangles (or in order words, continuous piecewise linear
# polynomials on triangles).
#
# Next, we use this element to initialize the trial and test functions
# ($u$ and $v$) and the coefficient functions ($f$ and $g$):
coord_element = element("Lagrange", "triangle", 1, shape=(2,))
mesh = Mesh(coord_element)
V = FunctionSpace(mesh, e)
u = TrialFunction(V)
v = TestFunction(V)
f = Coefficient(V)
g = Coefficient(V)
kappa = Constant(mesh)
# Finally, we define the bilinear and linear forms according to the
# variational formulation of the equations:
a = kappa * inner(grad(u), grad(v)) * dx
L = inner(f, v) * dx + inner(g, v) * ds
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