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Dec 10, 2018 at 16:47 comment added DCTLib @zhouziyao Actually, this answer contains exactly the proof idea, just written down slightly informally (but with all non-trivial steps included). Now, you only need to put this into formal notation and you have a proof.
Dec 9, 2018 at 17:52 comment added zhou ziyao Can u be more specific? I know the meaning of the formula. But i dont know how to prove second part from the first one. Thx
Dec 9, 2018 at 15:55 history answered David Richerby CC BY-SA 4.0