**Geometric Structures: Practice 8: Extensions of Finite Fields**

prepared by:

Joseph Malkevitch

Department of Mathematics and Computing

York College (CUNY)

Jamaica, New York 11451

email:

__malkevitch@york.cuny.edu__

web page:

__http://york.cuny.edu/~malk/__

1. Write down the multiplication table and addition table for the integers modulo 3, otherwise known as Z_{3}. Use the symbols 0, 1, and 2 for the elements of Z_{3}. (Hint: Each table should have 9 entries)

2. Write down all the quadratic polynomials whose coefficients are in Z_{3}. For each of these polynomials check which if any of the numbers 0, 1, and 2 are roots of the polynomial.

3. Verify that the polynomial x^{2} + x + 2 = 0 has no roots in Z_{3}. Hence, we can create a new finite number system (finite field) with 9 elements where each element has the form a + b Δ where Δ^{2} = - Δ - 2 = 2Δ + 1, and a and b are elements of Z_{3}. Some examples of these 9 numbers are: 0, 2, 1 + Δ, and 1 + 2Δ. This finite arithmetic is called GF (3^{2} ).

Construct an addition and multiplication table for these 9 numbers. (Hint: These tables will each have 81 entries.)

Comment: One can use the finite field that you constructed above to construct a finite affine geometry (where there are unique parallels) with exactly 81 points and 90 lines. Each line will contain exactly 9 points. Ten lines will go through every point The points will have the form (x, y ) where x and y are elements of GF (3^{2} ), and the lines will have the form ax + by + c = 0 where a, b, and c are not all 0 and are members of GF (3^{2} ). One can then construct a finite projective plane with 91 points and 91 lines in the usual way by introducing homogeneous coordinates.

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