The non‐real spectrum of a singular indefinite Sturm–Liouville operator with regular left endpoint

We provide bounds on the non‐real spectra of indefinite Sturm–Liouville differential operators of the form ( Af )( x ) = sgn( x )(− f ″( x ) + q ( x ) f ( x )) on the interval [ a , ∞), −∞ < a < 0, with real potential q ∈ L 1 ( a , ∞). The bounds depend only on the L 1 ‐norm of the negative part of q and the boundary condition at the regular endpoint a .