Theorem (The Spectral Theorem, Part I)
The eigenvalues of every Hermitian matrix H are real.
Proof.
Choose a unit vector v ā ĘH(Ī»). Then
Ī» = Ī» ā āØv, vā© = āØHv, vā©
= āØv, Ī» ā vā© = āØĪ» ā v, vā©
= āØv, Hvā© = Ī»Ģ ā āØv, vā©
= āØH*v, vā© = Ī»Ģ
So Ī»Ģ = Ī», which means Ī» ā ā.