For a polynomial P = sum_(k = 0)^n a_k z^k, several classes of norms are commonly defined. The l_p-norm is defined as left double bracketing bar P right double bracketing bar _p congruent ( sum_(k = 0)^n ( left bracketing bar a_k right bracketing bar )^p)^(1/p) for p>=1, giving the special cases left double bracketing bar P right double bracketing bar _1 | = | sum_j left bracketing bar a_k right bracketing bar left double bracketing bar P right double bracketing bar _2 | = | sqrt( sum_k ( left bracketing bar a_k right bracketing bar )^2) left double bracketing bar P right double bracketing bar _∞ | = | max_k left bracketing bar a_k right bracketing bar .