In gnuplot, the /
operator with integer operands does integer division, like Python's //
operator. So your 5/3
exponent evaluates simply to 1
, and likewise 2/3
is 0
.
If you want a fractional answer, use floating-point arguments:
f(x)=2*x**(5.0/3.0) + 5*x**(2.0/3.0)