Simplify the following natural logarithm with the non-zero constant $a$. \[ \ln \left(e^{-1 / a}\right) \] Answer:

Step 1 ：Given the natural logarithm \(\ln \left(e^{-1 / a}\right)\).

Step 2 ：The natural logarithm and the exponential function are inverse functions. Therefore, the natural logarithm of an exponential function simplifies to the exponent of the exponential function.

Step 3 ：In this case, the exponent is \(-1/a\).

Step 4 ：So, the simplified form of the given natural logarithm is \(\boxed{-\frac{1}{a}}\).