The Exponential function
The exponential function is the function whose derivative is equal to its equation. In other words:

Because of this special property, the exponential function is very important in mathematics and crops up frequently.
Like most functions, the exponential has an inverse function, which is ln x (pronounced 'log x').

The Natural Logarithm
ln x is also known as the natural logarithm. The derivative of ln x is 1/x.
It therefore follows that the integral of 1/x is ln x + c .