... v´’oi mo .i x ∈ X ta c´ox =∞n=1x, enen.Khi ¯d´olimn→∞Pnx = limn→∞nk=1x, ekek=∞n=1x, enen= x = Ix, ∀x ∈ X.Vˆa.y d˜ay {Pn} hˆo .i tu.¯d´ˆen I. Gi’a s’’u d˜ay {Pn} hˆo .i tu.¯d`ˆeu ¯d´ˆen I. ... Khi ¯d´olimn→∞Axn, u = y, u (i) ∀u ∈ X. M˘a.t kh´ac, do phi´ˆem h`am c’ua ¯d`ˆe b`ai liˆen tu.c nˆenlimn→∞Axn, u = Ax, u (ii)3∀u ∈ X. T`’u (i) v`a (ii), ta suy ray, u = Ax, u ⇔ y ... Ch´’ung minh r`˘ang d˜ay {Pn}nhˆo .i tu.¯di’ˆem ¯d´ˆen to´an t’’u ¯d`ˆongnh´ˆat I nh’ung khˆong hˆo .i tu.¯d`ˆeu ¯d´ˆen I. Gi’aiPnl`a ph´ep chi´ˆeu tr’u.c giao lˆen khˆong gian con tuy´ˆen t´ınh L{e1,...