  A variational problem for three-dimensional (3D) classical lattice models is
considered with trial state given by two-dimensional (2D) uniform product of
local variational weights. This approach, the tensor product variational
approach (TPVA), has been applied to 3D classical models (the Ising and the
Potts models). We consider a way of estimating the magnetic critical exponent
$\beta$ for the simple 3D Ising model assuming a functional form of the
spontaneous magnetization in the off critical region, where the TPVA provides
reliable data.
