In the last chapter, we inverted the Stürm - Liouville differential operator to obtain the inverse in the case where λ was not an eigenvalue. We can now determine the eigenfunctions of this operator form a complete orthonormal set in the Hilbert Space L2([a,b])https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315166209/a846e294-cb52-49dd-b4fd-176f6c2345a0/content/ieq2338.tif"/>.