= Single free node on a four-triangle mixed-boundary mesh
{title2=$2U_B=1/2,\quad U_B=1/4$}
Triangulate the region $0\leq x,y\leq2$, $x+y\geq2$ by $x=1$, $y=1$, and $x+y=3$. Impose zero Dirichlet data on the horizontal and vertical exterior edges and zero Neumann data on the sloping edge. Only the midpoint $B=(1,1)$ of the sloping edge remains free in the continuous piecewise linear space. For source one, its stiffness entry is $1/2+1/2+1=2$, while its load is three triangle contributions of $1/6$. The single free value is therefore $1/4$.
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