Given,
PT=aP+(a−1)I......(1)
Taking transpose on both sides we get,
(PT)T=(aP+(a−1)I)T
⇒P=aPT+(a−1)I.......(2)
Subtract equation (1) and (2),
⇒PT−P=a(P−PT)
⇒P=PT as a=−1
From equation (1),
P=aP+(a−1)I
⇒P=−I
Taking adjoint on both sides,
⇒AdjP=Adj(−I)
⇒∣AdjP∣=∣Adj(−I)∣=∣−I∣2
⇒∣AdjP∣=1