Find the range of the following real valued function;
f ( x ) = 2 ⌊ x ⌋ 2 x
Notation : ⌊ ⋅ ⌋ denotes the floor function .
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Simple standard approach.
f ( x ) = 2 ⌊ x ⌋ 2 x = 2 x − { x } 2 x = 2 { x } 2 x 2 x = 2 { x } where {x} is the fractional part of x .
As 0 ≤ { x } < 1 . The range of f ( x ) is [ 1 , 2 ) .
x can be written as ⌊ x ⌋ + y where y lies between 0 and 1 . It can be 0 when x is an integer. But it can never be 1 . So, 2 ⌊ x ⌋ cancels from the numerator and the denominator. And what we are left is 2 y where 0 ≤ y < 1 . So minimum value is 2 0 = 1 . Maximum value is something less than 2 1 = 2 .
So, the answer is [ 1 , 2 ) .
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We can rewrite the above function as;
f ( x ) = 2 x − ⌊ x ⌋ = 2 { x }
Where { x } denotes the fractional part of x . Now since the range of the fractional part of x is [ 0 , 1 ) we can figure out the range of f ( x ) .
The minimum value of the function would be: f ( 0 ) = 2 0 = 1 .
The maximum value of the function would be: f ( 0 . 9 9 9 . . . ) = 2 0 . 9 9 9 . . . → 2 .
That is the value tends to 2 but never quite reaches it. Thus the range of the function is [ 1 , 2 ) , the open bracket denotes that it never equals 2.