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# Routines for Conway and pseudo-Conway polynomials.

AUTHORS:

- David Roe
- Jean-Pierre Flori
- Peter Bruin

class `sage.rings.finite_rings.conway_polynomials.``PseudoConwayLattice`(p, use_database=True)

  Bases: `sage.structure.sage_object.SageObject`

  A pseudo-Conway lattice over a given finite prime field.

  The Conway polynomial \(f_n\) of degree \(n\) over \(\Bold{F}_p\) is defined by the following four conditions:

  - \(f_n\) is irreducible.
  - In the quotient field \(\Bold{F}_p[x]/(f_n)\), the element \(x\bmod f_n\) generates the multiplicative group.
  - The minimal polynomial of \((x\bmod f_n)^{\frac{p^n-1}{p^m-1}}\) equals the Conway polynomial \(f_m\), for every divisor \(m\) of \(n\).
  - \(f_n\) is lexicographically least among all such polynomials, under a certain ordering.

  The final condition is needed only in order to make the Conway polynomial unique. We define a pseudo-Conway lattice to be any family of polynomials, indexed by the positive integers, satisfying the first three conditions.

  INPUT:

  - `p` – prime number
  - `use_database` – boolean. If `True`, use actual Conway polynomials whenever they are available in the database. If `False`, always compute pseudo-Conway polynomials.

  EXAMPLES:

      sage: from sage.rings.finite_rings.conway_polynomials import PseudoConwayLattice
      sage: PCL = PseudoConwayLattice(2, use_database=False)
      sage: PCL.polynomial(3)
      x^3 + x + 1

  `check_consistency`(n)

    Check that the pseudo-Conway polynomials of degree dividing \(n\) in this lattice satisfy the required compatibility conditions.

    EXAMPLES:

        sage: from sage.rings.finite_rings.conway_polynomials import PseudoConwayLattice
        sage: PCL = PseudoConwayLattice(2, use_database=False)
        sage: PCL.check_consistency(6)
        sage: PCL.check_consistency(60)  # long

  `polynomial`(n)

    Return the pseudo-Conway polynomial of degree \(n\) in this lattice.

    INPUT:

    - `n` – positive integer

    OUTPUT:

    - a pseudo-Conway polynomial of degree \(n\) for the prime \(p\).

    ALGORITHM:

    Uses an algorithm described in [HL99], modified to find pseudo-Conway polynomials rather than Conway polynomials. The major difference is that we stop as soon as we find a primitive polynomial.

    REFERENCE:

    [HL99] | L. Heath and N. Loehr (1999). New algorithms for generating Conway polynomials over finite fields. Proceedings of the tenth annual ACM-SIAM symposium on discrete algorithms, pp. 429-437.

    EXAMPLES:

        sage: from sage.rings.finite_rings.conway_polynomials import PseudoConwayLattice
        sage: PCL = PseudoConwayLattice(2, use_database=False)
        sage: PCL.polynomial(3)
        x^3 + x + 1
        sage: PCL.polynomial(4)
        x^4 + x^3 + 1
        sage: PCL.polynomial(60)
        x^60 + x^59 + x^58 + x^55 + x^54 + x^53 + x^52 + x^51 + x^48 + x^46 + x^45 + x^42 + x^41 + x^39 + x^38 + x^37 + x^35 + x^32 + x^31 + x^30 + x^28 + x^24 + x^22 + x^21 + x^18 + x^17 + x^16 + x^15 + x^14 + x^10 + x^8 + x^7 + x^5 + x^3 + x^2 + x + 1

`sage.rings.finite_rings.conway_polynomials.``conway_polynomial`(p, n)

  Return the Conway polynomial of degree \(n\) over `GF(p)`.

  If the requested polynomial is not known, this function raises a `RuntimeError` exception.

  INPUT:

  - `p` – prime number
  - `n` – positive integer

  OUTPUT:

  - the Conway polynomial of degree \(n\) over the finite field `GF(p)`, loaded from a table.

  Note

  The first time this function is called a table is read from disk, which takes a fraction of a second. Subsequent calls do not require reloading the table.

  See also the `ConwayPolynomials()` object, which is the table of Conway polynomials used by this function.

  EXAMPLES:

      sage: conway_polynomial(2,5)
      x^5 + x^2 + 1
      sage: conway_polynomial(101,5)
      x^5 + 2*x + 99
      sage: conway_polynomial(97,101)
      Traceback (most recent call last):
      ...
      RuntimeError: requested Conway polynomial not in database.

`sage.rings.finite_rings.conway_polynomials.``exists_conway_polynomial`(p, n)

  Check whether the Conway polynomial of degree \(n\) over `GF(p)` is known.

  INPUT:

  - `p` – prime number
  - `n` – positive integer

  OUTPUT:

  - boolean: `True` if the Conway polynomial of degree \(n\) over `GF(p)` is in the database, `False` otherwise.

  If the Conway polynomial is in the database, it can be obtained using the command `conway_polynomial(p,n)`.

  EXAMPLES:

      sage: exists_conway_polynomial(2,3)
      True
      sage: exists_conway_polynomial(2,-1)
      False
      sage: exists_conway_polynomial(97,200)
      False
      sage: exists_conway_polynomial(6,6)
      False

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