Complex sentence: When he introduced surjunctive groups in 1973, Gottschalk observed that there were no known examples of non-surjunctive groups; as of 2014, it is still unknown whether every group is surjunctive.
Split into simpler sentences: When he introduced surjunctive groups in 1973, Gottschalk observed that there were no known examples of non-surjunctive groups. As of 2014, it is still unknown whether every group is surjunctive.