Nun, det(A-λE)=0 liefert für eine sym Matrix (aij) (a12=a21)
\( \lambda^{2} - \; \lambda (a11+a22) \; + a11 \; a22 -a12^{2} = 0\)
===>
\(\lambda = \frac{(a11 + a22) \underline{+} \sqrt{ (a11+a22)² -4(a11 a22 - a12² )}}{2}\)
===>√
\(a11^{2} - 2 \; a11 \; a22 + a22^{2}+ 4 \; a12^{2} \)
===>
\(\left(a11 - a22 \right)^{2} + 4 \; a12^{2} > 0\)
===>
λ∈ℝ