Definition 13.1.14 (Norm in $\R^{n}$).label A norm in $\R^{n}$ is $\norm{\cd}{}:\R^{n}\to[0,\infty)$ such that

  1. (1)

    $\norm{x}{}=0\iff x=0$

  2. (2)

    $\norm{\lam x}{}=\abs{\lam}\norm{x}{}$ for all $\lam\in\R$

  3. (3)

    $\norm{x+y}{}\leq\norm{x}{}+\norm{y}{}$

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