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The question is exercise 1.9 from Arora-Barak's book Computational Complexity — A Modern Approach:

Define a RAM Turing machine to be a Turing machine that has random access memory. We formalize this as follows: The machine has an infinite array A that is initialized to all blanks. It accesses this array as follows. One of the machine's work tapes is designated as the address tape. Also the machine has two special alphabet symbols denoted by R and W and an additional state we denote by q_access. Whenever the machine enters q_access, if its address tape contains 'i'R (where 'i' denotes the binary representation of i) then the value A[i] is written in the cell next to the R symbol. If its tape contains 'i'Wa (where a is some symbol in the machine's alphabet) then A[i] is set to the value a.

Show that if a Boolean function $f$ is computable within time $T(n)$ (for some time constructible $T$) by a RAM TM, then is is in $\mathrm{DTIME}(T(n)^2)$.

The trivial solution by using an additional tape recording pairs (address,value) turns out to be in $\mathrm{DTIME}(T(n)^3)$, since that tape can be of size $O(T(n)^2)$ with $O(T(n))$ pairs while the address of each pair can be of size $O(T(n))$.

Kaveh
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1 Answers1

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You write in the comments:

Since the address should be written in the tape by a $T(n)$-time Turing Machine, the size (i.e. string length) of the address can not exceed $O(T(n))$.

Can you use a similar argument to improve the bounds

[The] tape can be of size $O(T(n)^2)$ with $O(T(n))$ pairs while the address of each pair can be of size $O(T(n))$.

you mention in the question? You may need to remember what operations are possible in constant time on the RAM, that is using the precise definition the authors use.

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Raphael
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