... MQt 56 tinh cha't kh6ng gian Hilbert co b x=:L(x,X)Xj j =1 , 'r/XEX co c II ~ II = I I(x,x )1 j =1 'v'x EX 1. 8 Menh de 1. 8: Cho M hI mQtkh6ng gian dong cua mQtkh6ng gian Hilbert X , {Uj} j e J C ... (x,a), VxEXvallfll=llall 1. 11 £>inh IV1 .11 : (lax - Milgram) Cho f la mQtd""ngsong tuyen Hnh d6i xung, lien tl,lctren kh6ng 'gian Hilbert X cho: ~ a>O : f (x, x) :2:a II x 11 2 (buc) ... Vx E X jeJ 1. 9 £>inh Ii 1. 9: Cho {Xn}n la h~ dfty du kh6ng gian Hilbert X Neu day 56 eN {~n}n e N co thoa I~~ j =1 hQi t~ thl se co x EX nhat thoa (X, Xj)= ~j, 'r/ j EN 1. 10 £>inh Iv 1. 10: (Riesz)...