Abstract
I love the Hahn-Banach theorem. I love it the way I love Casablanca and the Fontana di Trevi. It is something not so much to be read as fondled. What is “the Hahn-Banach theorem?” Let f be a continuous linear functional defined on a subspace M of a normed space X. Take as the Hahn-Banach theorem the property that f can be extended to a continuous linear functional on X without changing its norm. Innocent enough, but the ramifications of the theorem pervade functional analysis and other disciplines (even thermodynamics!) as well. Where did it come from? Were Hahn and Banach the discoverers? The axiom of choice implies it, but what about the converse? Is Hahn-Banach equivalent to the axiom of choice? (No.) Are Hahn-Banach extensions ever unique? They are in more cases than you might think, when the unit ball of the dual is “round,” as for `p with 1 1, for 1 < p < ∞, despite the topologies being identical. The cubic nature of the unit ball does not suffice, however—if Y = c0, the extendibility dies. This article traces the evolution of the analytic form as well as subsequent developments up to 2004.
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