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Possible Duplicate:
When do the multiples of two primes span all large enough natural numbers?

If $m,n$ are positive integers and $(m,n)=1$.

What's the largest integer $N$ that cannot be expressed as $N=am+bn$, where $a$ and $b$ are nonnegative integers?

The answer is $N=mn-m-n$. How to prove it?

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    so shall we close it.2011-01-30

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