For energy values inside the well (E < 0), we get: cos ⁡ ⁡ ( k a ) = cos ⁡ ⁡ ( β β b ) cosh ⁡ ⁡ [ α α ( a − − b ) ] − − β β 2 − − α α 2 2 α α β β sin ⁡ ⁡ ( β β b ) sinh ⁡ ⁡ [ α α ( a − − b ) ] , with α α 2 = 2 m | E | ℏ ℏ 2 and β β 2 = 2 m ( V 0 − − | E | ) ℏ ℏ 2 .