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# Class Note for CMPSCI 601 at UMass(19)

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Date Created: 02/06/15
CMPSCI 601 Recall From Last Time Lecture 5 Turing Machines M QE6s 6 QXE gt QUhgtltEgtlte gt Def Function f is recursive iff it is computed by a TM f may be total or partial Def A set S is recursive iff its characteristic function X5 is a recursive function Recursive is the set of recursive sets A set S is recursively enumberable re iff its partial characteristic function p 5 is a recursive function re is the set of re sets Th Recursive re c0re CMPSCI 601 Palindromes Lecture 5 De nition 51 A string 11 E 2 is a palindrome iff it is the same as its reversal ie w 2 MB A Examples of palindromes 0 101 0 1 10100101 1 0 ABLE WAS I ERE I SAW ELBA 0 AMANAPLANACANALPANAMA Fact 52 T he set OfPALINDROMES over a xed alpha bet E is contextfree but not regular Proposition 53 The set ofPALINDROMES over a xed alphabet E is a recursive set Proof ABLEELBALJ A Fact 54 Time 0n2 is necessary and su icient for a one tape Turing machine to accept the set PALINDROMES Proof Time 0n2 su ices One way to see this is to do problems 284 285 from P A CMPSCI 601 MuliTape Turing Machines Lecture 5 De nition 55 A ktape Turing machine M Q E 6 8 Q nite set of states 8 E Q E nite set of symbols 6Q X we gt Q u W x 2 x 957 k Proposition 56 PALINDROMES can be accepted in DTIME on a 2tape TM Proof that PALINDROMES e DTIMEM ABLEELBALJ Eu DABLEELBALJ IgtLJ CMPSCI601 DTIME and DSPACE Lecture 5 De nition 57 A set A g 2 is in DTIMEtn iff there exists a deterministic multitape TM M and a constant c such that LA 2 M E w E 2 1 MW 1 and 2 V11 6 2 halts Within 01 steps De nition 58 A set A Q 2 is in DSPACEsn iff there exists a deterministic multitape TM M and a constant c such that lA M and 2 V11 6 2 uses at most 01 worktape cells Note The input tape is readonly and not counted as space used Otherwise space bounds below n would rarely be useful But in the real world we often want to limit space and work with readonly input A Example PALINDROMES e DTIMEM DSPACEM In fact PALINDROMES E DSPACElog CMPSCI601 FDTIME and FDSPACE Lecture 5 De nition 59 f 2 gt 2 is in FDTIMEtn iff there exists a deterministic multitape TM M and a constant c such that 1 f MM 2 V11 6 2 halts within 01 steps 3 3 1112100 ie f is polynomially bounded A De nition 510 f 2 gt 2 is in FDSPACE8n iff there exists a deterministic multitape TM M and a constant c such that 1f MM 2 V11 6 2 uses at most 01 worktape cells 3 3 1112100 ie f is polynomially bounded Input tape is readonly Output tape is writeonly Neither is counted as space used A Example Plus 6 FDTIMEn Times 6 FDTIMEn2 8 CMPSCI 601 L P and PSPACE Lecture 5 L E DSPACE10gn P E DTIMEn0lt1gt E CCJOIDTIMEW39 PSPACE 2 DSPACEn0lt1gt 2 OGIDSPACEW Theorem 511 For any functions tn 2 n 300 2 log n we have DTIMEtn Q DSPACEtn DSPACE8n Q DTIME20SH Proof Let M be a DSPACEsn TM letw E 2 letn M has k tapes and uses at most 08n worktape cells M has at most IQ n 090 2k 1216300 lt 2H4 possible con gurations Thus after 21M steps M must be in an in nite loop A Corollary 512 L g P g PSPACE 10 CMPSCI601 PALINDROMES E L LectureS EABLEELBALJ EILJ Using Olog n workspace we can keep track of and ma nipulate two pointers into the input 11 CMPSCI 601 DTIME versus RAMTIME Lecture 5 RAM Random Access Machine Memory F r0 r1 r2 r3 r4 n K program counter r0 accumulator Instruction Operand Semantics READ jltjlj Torrjlnjlj STORE j Tj 73 I my 2 TO ADD 3 th j Torrorjlmlj SUB lejlj T03T0 7quotj17quotrjlj HALF r0 2 LroQj JUMP j 1e j JPOS j if m gt 0 then 1e j JZERO j if m 0 then 1e 2 j HALT 1c 2 0 12 Theorem 513 DTIMEtn Q RAMTIMEtn Q DTIMEtn3 Proof Memorize program in nite control Store all registers on one tape Igt110101101010111O 7 K T0 T5 T11 Store workspace for calculations on second tape gt100lOlllJ Is A Use the third tape for moving over sections of the rst tape IgtO10110101011101J T0 T5 T11 Each register contains at most n tn bits The total number of tape cells used is at most MW WU 0tn2 Each step takes at most 0 steps to simulate A 13 CMPSCI 601 Nondeterministic TM Lecture 5 Nondeterrninistic Turing Machines choose one of two possible moves each step guesstm S g q 0 1 1 g7LJ7 iQ7LJ7 8707 gt 18717 gt Igt s1gt gt comment 9 or q guess 0 or 1 the rest 0 Write down an arbitrary string 9 E 0 1 the guess 0 Proceed With the rest of the computation using 9 if desired 0 Accept iff there exists some guess that leads to accep tance 14 1 971 J7 IQ71 J7 8707 gt 18717 gt Igt sIgt gt comment 9 orq guess 0 orl the rest 8 Eu 8 Igt1J g IgtLJ S IgtOLJ g IgtOLJ 81gt01LJ gIgtOlLJ 31gt011LJ glgt011LJ 81gt0110 1LJ qIgt0110 1LJ 15 De nition 514 The set accepted by a NTM N N E w E 2 1 some run of N halts with output 1 The time taken by N on w E N is the number of steps in the shortest computation of N that accepts A 9 n AU ooooooooooooowooooooooo tn 16 CMPSCI 601 NTIME and NP Lecture 5 NTIMEtn E probs accepted by NTMs in time Otn NP E NTIMEn0lt1gt E 39EJOINTIMEW Theorem 515 For any function DTIMEtn g NTIMEtn g DSPACEtn Recall DSPACEtn g DTIME20lttltngtgt Corollary 516 L C P g NP g PSPACE Corollary 517 The de nition of Recursive and re are unchanged if we use n0ndeierminisiic instead of deter ministic Turing machines 17 mm Arlthmetlc H1erarchy W c0re Recursive 139 e re complete Primitive Recursive EXPTIME PSPACE c0NP PolynomialTime Hierarchy complete c0NP NP NP 1 c0NP NP complete quottruly feasiblequot NC NC2 logCFL SAC NSPACE log n 3 DSPACE log n Regular LogarithmicTime Hierarchy 18

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