Volume of $E$ in $\mathbb{R}^4$ - AMM

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SUMMARY

The volume of the set $E$ in $\mathbb{R}^4$ consists of all real 4-tuples $(a, b, c, d)$ satisfying the inequality $(ax+by)^2+(cx+dy)^2 \le x^2+y^2$ for all $x, y \in \mathbb{R}$. This condition defines a geometric constraint that can be interpreted in terms of linear transformations and their effects on the unit circle in $\mathbb{R}^2$. The volume of $E$ can be computed using techniques from multivariable calculus and linear algebra, specifically involving the properties of norms and quadratic forms.

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  • Basic concepts of geometric interpretation of inequalities
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MountEvariste
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Let $E$ be the set of all real $4$-tuples $(a, b, c, d)$ such that if $x, y \in \mathbb{ R}$, then:
$(ax+by)^2+(cx+dy)^2 \le x^2+y^2$.
Find the volume of $E$ in $\mathbb{R}^4$.​

Source: AMM.
 
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Hint:

Show that $E$ is defined by the inequality $a^2 +b^2 +c^2 +d^2 \leqslant 1+(ad −bc)^2$ with $a^2 +c^2 \leqslant 1$ and $b^2 +d^2 \leqslant 1$.
 

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