>> why is f(0) in an ll test = 4
The value of f(0) must be such that f(0)-2 is a quadratic residue mod Mp
and f(0)+2 is a quadratic nonresidue mod Mp. 4 is the smallest value
which works for all p; 10 is the next, followed by 52. You could use
f(0) = 3 provided that 5 is a quadratic nonresidue mod Mp, which will be
the case when p = 3 mod 4, but 3 will not work when p = 1 mod 4. There
are some numbers, e.g. 6, which never work. The precise reasoning behind
this is a bit too complicated for a short reply.
Regards,
Bill
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