In signum and greatest integer function, why does the line starts with a large dot???????

The function f: RR defined by


is called the signum function. The domain of the signum function is R and the range is the set 


{−1, 0, 1}. The graph of the signum function is given below.


In the graph, f(x) is represented along y-axis.

Now, from the definition of signum function, it is clear that value of f(x)=0 only when x=0.

When we take x>0, value of the function becomes 1 and when we take x<0, the value of f(x) 

becomes -1. To represent it on the graph, we use a small circle at x=0,y=1 and x=0, y=-1 to 

make ot clear that the value of function f(x) is 0 at x=0, as we have x>0, value of f(x) becomes 

1 and when x<0, value of f(x) becomes -1.



Greatest Integer Function

The function f: RR defined by f (x) = [x], xR assumes the value of the greatest integer, 

less than or equal to x. Such a function is called the greatest integer function. E.g., [x] = 3 

for 3 ≤ x < 4. The graph of the function is given below.




We know that the value of greatest integer function changes at integer points i.e. value of 

greatest integer of 1.0, 1.11, 1.12, 1.16, 1.2, 1.43, ….., 1.99 will be 1 but the value of 2.0 will be 

2 and remain 2 upto 2.99...

That's why we use small circles to show on the graph that the value of function f(x) is not 1 at 

x=2, it is 1 up to those values of x which are just less than2. Similarly, we represent on the 

graph that the value of f(x) is not 2 at x=3, it is 2 for the values of x which are just less than 3.


Hope you will get the point!

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Since in a signum function its range is {-1,0,1} for any value of domain , hence to show that range of signum function is {-1,0,1} and since it does not change so it represented by big dots in the graph

In greatest integer function image of a domain is the greatest integer smaller than or equal to the element domain so the element of domain lies between greatest smaller integer and the next integer to it ,so it is just like a interval therefore it is represented in the graph by a line and dot just like an interval

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