Apparently you guys use the notation ...weird. Anyway, use the dimensions theorem since, after all, alinear function is just a linear transformation from
a vector space , a field.
About the dimension: if the functional is the zero func. then___, but if it is now the zero func. then it automatically is ONTO, so___
Dimensions theorem: if is a linear transfromation between vectors spaces over the same field, and if , we get