# What is u-substitution?

In answers on this site often the word "u-substitution" appears. Why this is called u-substitution? Is it the same as integration by substitution?

(I never heard this term before and I'm not sure if we have it in my native language)

Edit: I myself find the question rather stupid. However, I constantly was thinking "maybe I'm missing something". From the comments I can see that I'm not the only one who gets confused if something simple like "integration by substitution" is called something else like "u-substitution". Especially, if there is "u-substitution" maybe there is "v-substitution" and "t-substitution", and ... . All things which I might never heard of.

So I guess I leave this question here for everybody else who does not know what u-substitution is.

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It seems to be American slang; I remember being confused by it too... –  Ｊ. Ｍ. May 8 '11 at 7:56
This question is answered by the last sentence of the introduction to the Wikipedia article, which is the third Google result (for me) for "u-substitution": en.wikipedia.org/wiki/Integration_by_substitution –  Qiaochu Yuan May 8 '11 at 8:23
@J.M. mathematics slang? Nice :) Soon we'll be calling $C^{\infty}$ functions "smoothies" and prime numbers "lil' ninjas" –  Stijn May 8 '11 at 8:28
"Smoothies" I can understand, but why "lil' ninjas"? –  yunone May 8 '11 at 8:45

I think it's because the letter $u$ is often used, for some reason, to denote the name of the function that is used in the substitution.