2. Department of Algorithms and Computation
School of Engineering Science
College of Engineering
University of Tehran
Winter 2019
Test Subject
Presentation by : Vahid Baghi
4. Time is Tape Dependent
Theorem : Let 𝑡 𝑛 be a function , where 𝑡 𝑛 > 𝑛 .Then every 𝑡 𝑛 time multitape Turing
machine has an equivalent 𝑂(𝑡2
(𝑛)) time single-tape Turing machine.
Transition
Logic
⊔
1
0 ∞
⊔
#
0
0
1
#
0
1
1
#
1
0
∞
⊔
0
1
1 ∞
⊔
0
0
1
Transition
Logic
110
5. Proper Complexity Functions
Definition
There exists a TM M that outputs
exactly 𝑓 𝑛 symbols on input 1𝑛
and runs in time O(𝑓 𝑛 + 𝑛) and
space O(𝑓 𝑛 )
f is a proper complexity function if :
∀𝑛 ∶ 𝑓 𝑛 ≥ 𝑓(𝑛 − 1)
For Example : log 𝑛
, 𝑛 , 𝑛2 , …
6. A complexity class is a set of classes of decision problems (or languages) with the same
worst-case complexity
Complexity Classes
DTIME or TIME is the computational resource of
computation time for a deterministic Turing
machine
DTIME
DSPACE
DSPACE or SPACE is the computational
resource describing the resource of memory
space for a deterministic Turing machine
7. Time Complexity Hierarchy
Theorem : for any 𝑡 𝑛 > 0 , there exists a decidable language
L ∉ 𝐷𝑇𝐼𝑀𝐸(𝑡 𝑛 )
Define 𝐿 = 𝑖 𝑀𝑖 does not accept i within 𝑡 𝑖 time }
Question : is L ∈ 𝐷𝑇𝐼𝑀𝐸 𝑡 𝑛 ?
Proof :
Assume (towards contradiction) L ∈ 𝐷𝑇𝐼𝑀𝐸 𝑡 𝑛
∃ a fixed K ∈ 𝑁 such that Turing machine 𝑀𝐾 decides L within time bound 𝑡 𝑛