... ˘ ∀u ∈ X Mat kh´c, phiˆm h`m cua dˆ b`i liˆn tuc nˆn a e lim Axn , u = Ax, u n→∞ (ii) ` ∀u ∈ X Tu (i) v` (ii) , ta suy a ’ y, u = Ax, u ⇔ ⇔ y − Ax, u = 0, ∀u ∈ X y − Ax = ⇔ y = Ax Do (x, y) = ... v` a o ’ ’’ ’ o a n Pn x = x, ek ek , x ∈X, n = 1, 2, k=1 ` ´ ´ ´ a ’’ ¯o ` ˘ l` d˜y ph´p chiiˆu truc giao Chung minh rang d˜y {Pn }n hˆi tu diˆ’m dˆn to´n tu dˆng a a e e a o ¯ e ¯e ’ ’...