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Thread: Banach Algebra

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    Banach Algebra

    Let $\displaystyle A$ be a Banach Algebra without unit and be embeddd into a unital Banach Algebra $\displaystyle A^+ $such that the elements in $\displaystyle A^+ $are of the form $\displaystyle (a,\alpha),$$\displaystyle a \in A$ and $\displaystyle \alpha \in \mathbb{C}.$

    Define the multiplication in $\displaystyle A^+ $ by $\displaystyle (a,\alpha)(b,\beta)=(ab+\alpha b + \beta a,\alpha \beta)$ and the involution by $\displaystyle (a,\alpha)^*=(a^*,\alpha^-) $where $\displaystyle \alpha^- $ is the conjugate of $\displaystyle \alpha$.

    It is well known that under the norm $\displaystyle \|(a,\alpha)\|=\|a\|+|\alpha|, A^+ $ is a Banach algebra. However, if the norm is defined as $\displaystyle \|(a,\alpha)\|=max\{\|a\|,|\alpha|\}$, is $\displaystyle A^+ $still a Banach Algebra?

    I try to prove that$\displaystyle \|(a,\alpha)(b,\beta)\| \le \|(a,\alpha)\|\|(b,\beta)\|$ does not hold for a specific element in $\displaystyle A^+. $However, I still do not get a correct one. Can anyone help?
    Last edited by problem; Aug 3rd 2010 at 09:44 PM.
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