a, x^7 + x^5 + 1
= (x^7 + x^6 + x^5) − (x^6 − 1)
= x^5 (x^2 + x + 1) − (x^3 − 1) (x^3 + 1)
= x^5 (x^2 + x + 1) − (x − 1) (^x2 + x + 1) (x^3 + 1)
= (x^2 + x + 1) (x^5 − (x − 1) (x^3 + 1))
= (x^2 + x + 1) (x^5 − (x − 1) (x^3 + 1))
= (x^2 + x + 1) (x^5 −x^4+x^3-x+1)