21-356 Lecture 3

# 21-356 Lecture 3 - and assume that x0 is an accumulation...

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Example 5 Let f ( x, y ):= ! 1 if y = x 2 ,x % =0 , 0 otherwise. Given a direction v =( v 1 ,v 2 ) , the line L through 0 in the direction v intersects the parabola y = x 2 only in 0 and in at most one point. Hence, if t is very small, f (0 + tv 1 , 0+ tv 2 )=0 . It follows that ! f ! v (0 , 0) = lim t ! 0 f (0 + tv 1 , tv 2 ) " f (0 , 0) t =l im t ! 0 0 " 0 t . However, f is not continuous in 0 ,s inc e f " x, x 2 # =1 & 1 as x & 0 , while f ( x, 0) = 0 & 0 as x & 0 . Exercise 6 f ( x, y \$ x 2 y x 4 + y 2 if ( x, y ) % =(0 , 0) , 0 if ( x, y )=(0 , 0) . Find all directional derivatives of f at 0 and prove that f is not continuous at 0 . 5

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The previous examples show that in dimension N ! 2 partial derivatives do not give the same kind of results as in the case N =1 .T oso lv eth i sp rob lem ,w e introduce a stronger notion of derivative, namely, the notion of di ! erentiability. We recall that a function T : R N " R is linear if T ( x + y )= T ( x )+ T ( y ) for all x , y # R N and T ( s x sT ( x ) for all s # R and x # R N .W r i t e x = ! N i =1 x i e i . Then by the linearity of T , T ( x T " N # i =1 x i e i \$ = N # i =1 x i T ( e i ) .
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21-356 Lecture 3 - and assume that x0 is an accumulation...

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