Revise your solution to Prob. 8.60 to apply the pib basis functions to the one-dimensional quartic oscillator with V = cx4. See Prob. 8.63 for hints. Take the "box" to extend from xr = -3.5 to 3.5, where xr is as found in Prob. 4.32. Increase the number of pib basis functions until the lowest three energy values remain stable to three decimal places. Compare the lowest three energies with those found by the Numerov method in Prob. 4.32. Check the appearance of the lowest three variational functions. Now repeat for the box going from xr = -4.5 to 4.5. For which box length do we get faster convergence to the true energies?