In keeping with the original question of this post, I believe there are some other concepts that need clarifying (not merely “infinity”.) What is a constant? What is a variable? These also need to be understood from context. From the question it sounds like the underlying context is the set of Real Numbers.
So, to be more clear, I will emphasize “real-valued”.
aaryan0077 said:
Infinity can't be a constant because ∞ ± k = ∞ , but a constant changes (its) value when something is added or subtracted.
A
real-valued constant changes when a nonzero number is added. The constant changes? Hmm... What is meant by this, I think, is that “The sum of a
real-valued constant and a nonzero number is different than the value of that constant.”
Hence:
Infinity is not a
real-valued constant. [Even when the set of Reals is extended to make the “Extended Reals”, infinity is still not a
real-valued constant.]
aaryan0077 said:
But infinity can't be variable because the definition of variable is
"A variable is a symbol that stands for a value that may vary" or stating in simple terms
"In mathematics, a changing quantity (one that can take various values) is variable"
A
real-valued variable is a variable that takes on various real-values.
Infinity never takes on a real-value.
Hence:
Infinity is not a
real-valued variable.
aaryan0077 said:
But infinity is not defined, so it can't vary with wrt to anything.
So what is it?
“not defined” is a bad choice of words. As has been mentioned before in this post, the context is important for knowing which “infinity” is being used; which definition is being used.
When dealing with real-valued numbers, and their functions, the term infinity is used to describe a process in which a varying real-value increases without bound.
“As x grows without bound” [Notation: x\rightarrow \infty ]
That is, for any real number b, x is “eventually” larger than b (and stays larger.) There is an implied process going on, namely that of x changing in value.
Infinity is also used in interval notation to represent an unbounded interval.
“All real numbers greater than 4” [Notation: \left(4,\infty\right) ]
Notice in the two examples above, the word “infinity” is not needed, nor is the symbol needed. It is for convenience so we need not always write “unbounded growth” or “unbounded interval”.
When dealing with sets other than the Real Numbers, the term “infinity” might not be used in these ways.