Linear Independence of two Functions

by mattmns
Tags: functions, independence, linear
 P: 1,119 Hello, there is this question in the book: --------- Consider the vector space of functions defined for t>0. Show that the following pairs of functions are linearly independent. (a) t, 1/t --------- So if they are linearly independent then there are a,b in R, such that at + b/t = 0 So if we differentiate both sides with respect to t we get $$a + \frac{-b}{t^2} = 0$$ which implies $$a = \frac{b}{t^2}$$ If we plug this into the first equation, we get $$\frac{b}{t^2}t + \frac{b}{t} = 0$$ so, $$\frac{2b}{t} = 0 => b = 0$$ If we plug this back to the first equation it follows that a = 0. So a = 0 and b = 0, and therefore the two functions, t and 1/t, are linearly independent. --------- Is that sufficient? Also, what if I just said something like let t = 1, you then get the equation a + b = 0 then let t = 2, and you get the equation 2a + b/2 = 0 And if you solve these equations simultaneously, it follows that a = 0 and b = 0 are the only solutions. Would that be sufficient, given that we know a = 0 and b = 0 are always solutions? Thanks!
 PF Patron Sci Advisor Thanks Emeritus P: 38,424 Yes, that's perfectly valid. Remember that saying at+ b/t= 0 means it is true for all t. The simplest thing to do is just choose two values for t so you have two equations to solve for a and b. Taking t= 1 tells us that a+ b= 1. Taking t= 2 gives a+ b/2= 0. Subtracting that from a+ b= 0, we have b/2= 0 so b=0. Putting b= 0 int a+ b= 0 gives a= 0 so the functions are linearly independent.
 P: 1,119 Awesome, thanks!

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