x^15+1
=(x^5)^3+1^3
=(x^5+1)(x^10-x^5+1)
=(x^5+x^4-x^4-x^3+x^3+x^2-x^2-x+x+1)(x^10-x^5+1)
=[x^4(x+1)-x^3(x+1)+x^2(x+1)-x(x+1)+(x+1)](x^10-x^5+1)
=(x+1)(x^4-x^3+x^2-x+1)(x^10-x^5+1)