Infinite Limit
A function f(x) has the limit +∞ when x 
a if there is a positive real number K > 0, that verifies that f(x) > k for all values close to a.
Minus Infinite Limit
A function f(x) has the limit -∞ when x a if there is a positive real number K < 0 and this verifies that f(x) < k for all values close to a.
