Solve the time independent Schr\"odinger equation for the potential
V(x)=−ℏ2/(ma2cosh2ax).
Hint: Introduce y=x/a, make the ansatz
ψ(y)=eikyφ(y),
and substitute z=tanhy. The resulting equation can be solved in elementary fashion by a power series together with a condition for the series to terminate. Discuss the solutions that occur.