Consider a square lattice in two dimensions having a
lattice constant of
*
a
*
with a crystal potential
*
U
*
(
*
x
*
,
*
y
*
)
= -4
*
U
*
cos(2
p
*
x
*
/
*
a
*
) cos(2
p
*
y
*
/
*
a
*
).
Using Fourier analysis and by determining which reciprocal lattice vectors
fulfill the condition for Bragg reflection of electron waves, find approximately
to first order the energy gap (
*
E
_{
g
}
*
) at the corner point (
p
/