Functions are not continuous in their work

Calculus Level 3

f ( x ) = { sin ( π 4 { x } ) if x is odd cos ( π 4 ( 1 { x } ) ) if x is even for x ( 0 , 4 ) f(x) = \begin{cases} \sin \left(\dfrac \pi 4 \{x\} \right) & \text{if }\lfloor x \rfloor \text{ is odd} \\ \cos \left(\dfrac \pi 4 \left(1-\{x\}\right)\right) & \text{if }\lfloor x \rfloor \text{ is even} \end{cases} \quad \text{for }x \in (0,4)

Find the number of points of continuity of f ( x ) f(x) when x x is an integer.

Notations:


The answer is 1.

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1 solution

Chew-Seong Cheong
Aug 24, 2018

Consider the transition from even x \lfloor x \rfloor to odd x \lfloor x \rfloor that is from 0 to 1 and from 2 to 3.

{ lim { x } 1 cos ( π 4 ( 1 { x } ) ) = cos 0 = 1 sin ( π 4 ( 0 ) ) = 0 1 \begin{cases} \lim_{\{x\} \to 1^-} \cos \left(\dfrac \pi 4(1-\{x\})\right) = \cos 0 = 1 \\ \sin \left(\dfrac \pi 4 (0) \right) = 0 \color{#D61F06} \ne 1 \end{cases} \implies no continuity.

Consider the transition from odd x \lfloor x \rfloor to even x \lfloor x \rfloor that is from 1 to 2.

{ lim { x } 1 sin ( π 4 { x } ) = sin π 4 = 1 2 cos ( π 4 ( 1 0 ) ) = cos π 4 = 1 2 \begin{cases} \lim_{\{x\} \to 1^-} \sin \left(\dfrac \pi 4 \{x\} \right) = \sin \dfrac \pi 4 \color{#3D99F6} = \dfrac 1{\sqrt 2} \\ \cos \left(\dfrac \pi 4(1-0)\right) = \cos \dfrac \pi 4 \color{#3D99F6} = \dfrac 1{\sqrt 2} \end{cases} \implies continuity at x = 2 x=2 .

Therefore, there is 1 \boxed 1 point of continuity when x x is an integer.

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