2000 Mathematics Subject Classification. Primary: 45E10; Secondary: 45G05. Key words and phrases. Bessel potential, integral equations, positive solutions, the regularity lifting theorem, integrability intervals, radial symmetry, decay rate. The first author is supported by the Scientific Research Fund of Hunan Provincial Education Department grant 16C0763 and 17C0754; The second author is supported by the NSF of China grant 11671086 and 11871208 and NSF of Hunan Province of China (No. 2018JJ2159); The third author is supported by the NSF of China grant 11771358. ∗ Corresponding author.