What is the characteristic equation for an=an−1+an−2+an−3a_n = a_{n-1} + a_{n-2} + a_{n-3}an=an−1+an−2+an−3?
r3+r2+r+1=0r^3 + r^2 + r + 1 = 0r3+r2+r+1=0
r3−r2−r−1=0r^3 - r^2 - r - 1 = 0r3−r2−r−1=0
r3−r2−1=0r^3 - r^2 - 1 = 0r3−r2−1=0
r3−r−1=0r^3 - r - 1 = 0r3−r−1=0