quartic polymomial with negative leading coefficient
scaled 1/x function with horizontal offset
Explanation:
The wiggles of section 1 can be attributed to a number of different functions. Perhaps the simplest is a 4th-degree polynomial. In order to have downward-trending end behavior, it would need to have a negative leading coefficient.
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The curve of section 2 looks like it might be a scaled and translated version of 1/x, or it could be an exponential function. The latter would be expected to approach the horizontal asymptote more quickly than shown here, so we prefer a version of 1/x.