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How do you determine if a quadratic polynomial is irreducible over a given field?

A quadratic polynomial over a field is irreducible if and only if it has no roots in that field. You can check this by evaluating the polynomial at all elements of the field. If none of the evaluations yield zero, the polynomial is irreducible.

Conditions

  • The polynomial is of degree 2.
  • The coefficients are in a field.

Reasoning, step by step

  1. Identify the quadratic polynomial and the field.
  2. Evaluate the polynomial at every element of the field.
  3. Check if any evaluation results in zero.
  4. If no roots are found, conclude the polynomial is irreducible.

Example

The narrator checks g(x)=x2+2x+2g(x) = x^2 + 2x + 2 over Z3\mathbb{Z}_3: g(0)=2g(0)=2, g(1)=5≡2g(1)=5\equiv 2, and g(2)=10≡1g(2)=10\equiv 1. Since none are zero, g(x)g(x) is irreducible.

Common misconceptions

  • Assuming a quadratic is irreducible just because it doesn't factor into integers, without checking the specific field.
  • Forgetting to check all elements of the finite field.

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