A remark on equivalent Rosser sentences

An oversight in Guaspari and Solovay’s “Rosser sentences” [D. Guaspari, R.M. Solovay, Rosser sentences, Annals of Mathematical Logic 16 (1) (1979) 81–99] is pointed out and emended. It concerns the premisses of their proof that there are standard proof predicates all of whose Rosser sentences are provably equivalent. The result holds up, but the premisses mentioned in the paper have to be strengthened somewhat.