McColm's conjecture [positive elementary inductions]
Yuri G. Gurevich, Neil Immerman, Saharon Shelah · 2002
G. McColm (1990) conjectured that positive elementary inductions are bounded in a class K of finite structures if every (FO+LFP) formula is equivalent to a first-order formula in K. Here (FO+LFP) is the extension of first-order logic with the least fixed point operator. We disprove the conjecture. Our main results are two model-theoretic constructions, one deterministic and the other randomized, each of which refutes McColm's conjecture.>