Update lecture_1.tex

Correction(O-notaion problem)
Merge pull request #4 from prattyboy/patch-2

Update lecture_1.tex
Update lecture_2.tex

Заменил (где заметил) – на —
Merge pull request #5 from airyplanet/(1-3)-corrections

Update lecture_2.tex исправлено тире
Update lecture_3.tex
Merge pull request #6 from airyplanet/patch-1

Update lecture_3.tex
Merge pull request #7 from hayer44/(1-3)-corrections

(1 3) corrections