From Wikipedia:
"A basis B of a vector space V over a field K is a linearly independent subset of V that spans (or generates) V.(1)
B is a minimal generating set of V, i.e., it is a generating set and no proper subset of B is also a generating set.(2)
B is a maximal set of linearly independent vectors, i.e., it is a linearly independent set but no other linearly independent set contains it as a proper subset."(3)
I tried to prove (1) => (3) => (2), to see that these are equivalent definitions, can you tell me if my proof is correct:
(1) => (3):
Let B be linearly independent and spanning. Then B is maximal: Let v be any vector in V. Then since B is spanning, ∃bi∈B,ki∈K:∑ni=1biki=v. Hence v−∑ni=1biki=0 and hence B∪{v} is linearly dependent. So B is maximal since v was arbitrary.
(3) => (2):
Let B be maximal and linearly independent. Then B is minimal and spanning:
spanning: Let v∈V be arbitrary. B is maximal hence B∪{v} is linearly dependent. i.e. ∃bi∈B,ki∈K:∑ibiki=v, i.e. B is spanning.
minimal: B is linearly independent. Let b∈B. Then b∉span(B∖{b}) hence B is minimal.
(2) => (1):
Let B be minimal and spanning. Then B is linearly independent:
Assume B not linearly independent then ∃bi∈B,ki∈K:b=∑ibiki. Then B∖{b} is spanning which contradicts that B is minimal.
Answer
The proof looks good (appart form the obvious mix up in the numbering). One thing which is not totally precise:
In your second proof you write
Let v∈V be arbitrary. B is maximal hence B∪{v} is linearly dependent. i.e. ∃bi∈B,ki∈K:∑ibiki=v, i.e. B is spanning.
To be precise you have ki∈K not all vanishing such that k0v+∑ikibi=0. Since B is linearly independent k0=0 implies ki=0 for all i, therefore k0≠0 and v is in the span of B.
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