Also, 625 = 54 = 252 is equivalent to saying log5 625 = 4 or
log25 625 = 2 On a slightly more mature note, fixing a  base b > 1, we may look at logarithm as
a function from positive real numbers to all real numbers This function, called the
logarithmic function, is defined by
logb : R+ → R
x → logb x = y   if by = x
As before if the base b = 10, we say it
is common logarithms and if b = e, then
we say it is natural logarithms Often
natural logarithm is denoted by ln