The two conditions, to figure out if a function is defined at an input value or not, are explained.

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A function `f(x)` is defined at the input value `x=a`, when `f(a)` evaluates to a defined value.

If `f(a)` evaluates to `oo` or `0/0`, then the function need to be further examined.

If the left-hand-limit and right-hand-limit take a defined value and are equal, then the function is defined at that input value.

This is understood from

• In an application scenario, the value is not exact and so the function will evaluate to a stable value given by limits.

• In an abstract scenario, the expected value at that input value can be taken to be the value of the function.

* A function is defined* at an input value, if the function evaluates to a definite value OR if the left-hand-limit and right-hand-limits are equal.

The word 'defined' means: give exactly; state or describe without any ambiguity.

**Function is defined: ** Given a function `f(x)`, the function is defined at `x=a`

• if `f(a)` is a defined value. OR

• if `f(a)` is an indeterminate value `0/0` or undefined large `oo`, then `lim_(x->a+)f(x)=lim_(x->a-)f(x)`

Difference between Defined and Continuous :

A function is defined if it evaluates to a definite value *OR* when is the function evaluates to indeterminate value `0/0`, then left-hand-limit and right-hand-limit are equal.

A function is continuous only when it evaluates to a definite value at the input value *AND* also left-hand-limit and right-hand-limit are equal to the definite value at the input value.

• For a function to be continuous at a input value, it has to be defined at that input value.

• If a function is defined at an input value, it does not imply it is continuous. It may be or may not be continuous.

• If a function is not defined at an input value, it implies it is not continuous.

*Solved Exercise Problem: *

If a function is not continuous at an input value, can the function be defined at that input value?

- Yes.
- Yes.
- No.

The answer is 'Yes.'. Theoretically a function can have a value `f(a)` different to the limits.

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