Because of its natural expression in terms of T and p.

因为G作为p和T的很自然的函数。

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They come from writing i prime and j prime in terms of i and j, then sticking it in an expression and identifying the new representation.

写出用 i 和 j 来表示 i' 和 j' 的式子,再代入分量的表达式,就能得到新的式子

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