However , in 1974 Perko discovered a counterexample called the Perko pair , a pair of knots listed as distinct in tables for many years that are in fact the same .
Little claimed in 1900 that the writhe of a reduced knot diagram is an invariant .
In 1974 Perko discovered a counterexample called the Perko pair , a pair of knots listed as distinct in tables for many years that are in fact the same .
Little claimed in 1900 that the writhe of a reduced knot diagram is an invariant .