Volpert, Vitaly, and Vitali Vougalter. “Solvability in the Sense of Sequences for Some Non-Fredholm Elliptic Problems”. Discontinuity, Nonlinearity, and Complexity, vol. 2, no. 4, Aug. 2026, pp. 389-9, https://doi.org/10.5890/DNC.2013.11.007.