Thursday, March 24, 2016

finite fields - Discrete logarithm - strange polynomials

If p is a prime number and ω is a fixed primitve root for Z/pZ, then we can define the discrete logarithm of x(Z/pZ)× as the unique number logωx between 1 and p1 such that ωlogωx=x.



logωx will be identified with its remainder class in Z/pZ.




I was interested in finding the interpolating polynomial in Z/pZ[x] of minimal degree for this logω function (for fun?) and looked at a few examples where ω=3 can be taken as a fixed primitive root. Some examples:



modulo 5, log3(x)=4x3+3x2+2x.



modulo 7, log3(x)=5x5+2x4+4x3+6x2+3x.



modulo 17, log3(x)=
10x15+16x14+3x13+11x12+14x11+12x10+13x9+9x8+5x7+6x6+4x5+7x4+15x3+2x2+8x.



I notice that the coefficients in each case are exactly the numbers 2 to p1 up to permutation. Is there something behind this? It seems highly improbable if it's a coincidence.

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