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We show that inverse square singularities can be treated as boundaryconditions containing rational Herglotz--Nevanlinna functions of the eigenvalueparameter with "a negative number of poles". More precisely, we treat in aunified manner one-dimensional Schr\"{o}dinger operators with either an inversesquare singularity or a boundary condition containing a rationalHerglotz--Nevanlinna function of the eigenvalue parameter at each endpoint, anddefine Darboux-type transformations between such operators. Thesetransformations allow one, in particular, to transfer almost any spectralresult from boundary value problems with eigenparameter dependent boundaryconditions to those with inverse square singularities, and vice versa.

Inverse square singularities and eigenparameter dependent boundary conditions are two sides of the same coin