#Hodge on #Lefschetz's paper: similar principles applied to algebraic surfaces:
if ω =/=0 #holomorphic form on an algebraic surface, then i: sqrt {-1}}\,\omega ^ {\bar {\omega } is + , i^2=/=0, must b. =>ω => =/=0 #cohomology class,so its periods=/=0
Fixed a question of #Severi

#Hodge on #Lefschetz's paper: similar principles applied to algebraic surfaces:
if ω =/=0 #holomorphic form on an algebraic surface, then i: sqrt {-1}}\,\omega ^ {\bar {\omega } is + , i^2=/=0, must b. =>ω => =/=0 #cohomology class,so its periods=/=0
Fixed a question of #Severi