Factorization, ladder operators and isospectral structures
  Using the modified factorization method employed by Mielnik for the harmonic oscillator, we show that isospectral structures associated with a second order operator $H$, can always be constructed whenever $H$ could be factored, or exist ladder operators for its eigenfunctions. Three examples are shown, and properties like completeness and integrability are discused for the general case. 
A. P\'erez-Lorenzana