Padding argument

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In computational complexity theory, the padding argument is a tool to conditionally prove that if some complexity classes are equal, then some other bigger classes are also equal.


The proof that P = NP implies EXP = NEXP uses "padding". by definition, so it suffices to show .

Let L be a language in NEXP. Since L is in NEXP, there is a non-deterministic Turing machine M that decides L in time for some constant c. Let

where 1 is a symbol not occurring in L. First we show that is in NP, then we will use the deterministic polynomial time machine given by P = NP to show that L is in EXP.

can be decided in non-deterministic polynomial time as follows. Given input , verify that it has the form and reject if it does not. If it has the correct form, simulate M(x). The simulation takes non-deterministic time, which is polynomial in the size of the input, . So, is in NP. By the assumption P = NP, there is also a deterministic machine DM that decides in polynomial time. We can then decide L in deterministic exponential time as follows. Given input , simulate . This takes only exponential time in the size of the input, .

The is called the "padding" of the language L. This type of argument is also sometimes used for space complexity classes, alternating classes, and bounded alternating classes.