This is a question from Stephan Belniuk's Stefan Bilaniuk's very good free online book "A Problem Course in Mathematical Logic"Logic:
Problem 18.6. Suppose Sigma$\Sigma$ is a recursive set of sentences of LN. Find a sentence of LN, which we'll denote by Con(Sigma), $Con(\Sigma)$, such that Sigma$\Sigma$ is consistent if and only if $A |- Con(Sigma)\vdash Con(\Sigma)$.
By Godel's second incompleteness theorem isn't this impossible?

