For decades, one classification problem - involving a particular set of infinitely large objects called torsion-free abelian groups (or TFABs) - stymied researchers. This problem was first raised in 1989 by the mathematicians Harvey Friedman and Lee Stanley in a paper that, according to Paolini, "introduced a new way of comparing the difficulties of classification problems for countable structures, indicating that some things are more complicated than others."