What is the characteristic equation for an=an−2a_n = a_{n-2}an=an−2?
r2−1=0r^2 - 1 = 0r2−1=0
r2+1=0r^2 + 1 = 0r2+1=0
r−1=0r - 1 = 0r−1=0
r2−r=0r^2 - r = 0r2−r=0