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Published online by Cambridge University Press: 20 November 2018
We study some geometric properties related to the set
    $${{\Pi }_{X}}\,:=\left\{ \left( x,\,{{x}^{*}} \right)\,\in \,{{\text{S}}_{X}}\,\times \,{{\text{S}}_{{{X}^{*}}}}\,:\,{{x}^{*}}\left( x \right)\,=\,1 \right\}$$
 $${{\Pi }_{X}}\,:=\left\{ \left( x,\,{{x}^{*}} \right)\,\in \,{{\text{S}}_{X}}\,\times \,{{\text{S}}_{{{X}^{*}}}}\,:\,{{x}^{*}}\left( x \right)\,=\,1 \right\}$$
obtaining two characterizations of Hilbert spaces in the category of Banach spaces. We also compute the distance of a generic element   $\left( h,\,k \right)\,\in \,H\,{{\oplus }_{2}}\,H$  to
 $\left( h,\,k \right)\,\in \,H\,{{\oplus }_{2}}\,H$  to   ${{\Pi }_{H}}$  for
 ${{\Pi }_{H}}$  for   $H$  a Hilbert space.
 $H$  a Hilbert space.