I Doi vdi may Turing khong tat dinh diing N, quat rinh tinh toan trgn moi t i i vao w duqc dien ra theo mot nhanh nao do ma may lua
2.1 Do phitc tap thdi gian cua cdc loai may Turing
mot luong thdi gian Idn nhat, vh ti6n hanh xac dinh do phiJc tap
thdi gian cua thuat toan.
Do luong thdi gian thuat toan can den trong trudng hop xau
nhat thudng dirge dien dat bdi mOt bigu thufc phiic tap, nen thong
thudng ta tim each udc luong no bcii bigu thiic don gian. Mat khac,
theo ygu cau, ta cung chi can biet duoc luong thdi gian khi tinh
toan trgn nhiing dau vao cd Idn. De dap ling nhang yeu cau thich
hop nay, khai niem 0-ldn thudng dude sii dung va to ra kha tien loi
trong viec dien ta do philc tap thdi gian cua thuat toan. Nhu vay, khi khong can thiet phai xac dinh ehinh xac, do phutc t^p thdi
gian cua thuat toan dugc thg hien mot each tuong doi bdi can tren
tiem cQ,n cua nọ
Nhu da thay, may Turing c6 kha nang dien ta mSt each t i mi cdc qua trinh tinh toan va dugc thiia nhan la mot mo hinh toan
hoc ciia thuat toan. Bdi vay, viec phan tich thuat toan doi vdi may
Turing se cho ta hiiu dugc mot each can ke vg ban chat cua cac
qua trinh tinh toan, va qua do khong chi giup ta tim dirge can tren
tiem can hoae xac dinh chinh xac do phiie tap thdi gian cua may
Turing ma eon ggi md cho ta cdi tien each tinh toan hoae Itfa chgn
mo hinh tinh toan khac hieu qua hon.
Nhiing y tudng nay se duge thg hien qua viec xem xet mgt vai
may Turing, dirge xay dung nham khang dinh ngon ngii cho trudc
= {Ór I I = 0,1, 2,... } trong Vi du 2.1.1. : • Dau tign, ta xac dinh can tren tiem can doi vdi do phufc tap thdi j
gian cua may Turing A ma ta vita xay dung trong Vi du 2.1.1. Qua viec mo ta may A ta nhan thay rang, qua trinh tinh toan cua A tren moi tir vao ly G {0,1}* dugc chia thanh hai cong doan. Cong \
doan 1 loai bo cac til who khong c6 dang Ól^ vdi i,j > 0. Cong doan 2 thue hien mot quy trinh lap trgn nhOng tir vao dang 0'1>,
n^oi bude lap bao gom viec xoa ky tu 0 dau ben trai va ky tu 1 dau j
ben phai cua tii trgn bang, va chap nhan neu va chi neu trgn bang \
124 Do phurc tap thcii gian Viec phan tich may Turing A dupe tien hhnh theo tijfng cong