# Hahn Banach theorem with no dominating sublinear functional

Let $V$ be a vector space and $M$ be subspace of it. If $f$ is a linear functional on $M$, is it possible to extend it to the whole space $V$? If we have a sublinear functional $p$ on $V$ dominating $f$ on $M$, then by Hahn Banach we know that there is an extension. I would like to know if the same statement is valid with out the hypothesis of domination sublinear functional. I would like to understand the role of sublinear functional in the proof of Hahn Banach. Thanks

• Sure it's possible, extend a basis of $M$ to a basis of $V$, and define the extension arbitrarily on the basis vectors not in $M$. The domination by a sublinear functional is something that makes the existence of an extension more difficult to prove. The important result is that often, you have a continuous extension. Aug 18, 2013 at 17:54
• what if $V$ is infinite dimensional and there is no basis? Aug 18, 2013 at 17:56
• @chandu1729 The axiom of choice (or Zorn's lemma if you prefer) allows you to always find bases of vector spaces. Again to echo Daniel, the crucial thing about Hahn-Banach is the relationship with continuity. Aug 18, 2013 at 17:59
• The nice thing about dominating the sublinear functional is that many times we can get information about the norm of the extension, about continuity, etc. An extension always exists, but doesn't always give all the information or characteristics we want. Aug 18, 2013 at 19:05
• Look for "Hamel basis", a term often used for infinite-dimensional spaces. Aug 18, 2013 at 19:15