How do I declare a static finite field Element when modulus is a prime power ?

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Simple answer:

Once you have z12 assigned to a variable you can use it directly:

sage: X = 16260673061341949275257563295988632869519996389676903622179081103440260644990*z12^9 + 11559732032986387107991004021392285783925812861821192530917403151452391805634*z12^3

To obtain the generator z12 of the finite field GF(q) you have basically two options.

The first option is to define the field and its generator at the same time instead: FF.<z12> = GF(q).

The second option is to retrieve z12 = FF.gen() after defining FF = GF(q).

These options are equivalent; the first is a SageMath-specific shorthand syntax, the second is standard Python.