 Let formula_23 be the torsion subgroup of formula_24.Let formula_25 be the quotient by torsion.If formula_20 is a surface, formula_27 is non-canonically isomorphic to formula_28, where formula_29 is the genus; more generally, formula_27 is non-canonically isomorphic to formula_31, where formula_32 is the first Betti number.