README.md

# CycPols — Module.

This package deals with products of Cyclotomic polynomials.

Cyclotomic numbers, and cyclotomic polynomials over the rationals or some cyclotomic field, are important in the theories of finite reductive groups and Spetses. In particular Schur elements of cyclotomic Hecke algebras are products of cyclotomic polynomials.

The type CycPol represents the product of a coeff (a constant, a polynomial or a rational fraction in one variable) with a rational fraction in one variable with all poles or zeroes equal to 0 or roots of unity. The advantages of representing as CycPol such objects are: nice display (factorized), less storage, fast multiplication, division and evaluation. The drawback is that addition and subtraction are not implemented!

This package uses the polynomials Pol defined by the package LaurentPolynomials and the cyclotomic numbers Cyc defined by the package CyclotomicNumbers.

The method CycPol(a::Pol) converts a to a CycPol by finding the largest cyclotomic polynomial dividing, leaving a Pol coefficient if some roots of the polynomial are not roots of unity.

julia> using LaurentPolynomials

julia> @Pol q
Pol{Int64}: q

julia> p=CycPol(q^25-q^24-2q^23-q^2+q+2) # a `Pol` coefficient remains
(q-2)Φ₁Φ₂Φ₂₃

julia> p(q) # evaluate CycPol p at q
Pol{Int64}: q²⁵-q²⁴-2q²³-q²+q+2

julia> p*inv(CycPol(q^2+q+1)) # `*`, `inv`, `/` and `//` are defined
(q-2)Φ₁Φ₂Φ₃⁻¹Φ₂₃

julia> -p  # one can multiply by a scalar
(-q+2)Φ₁Φ₂Φ₂₃

julia> valuation(p)
0

julia> degree(p)
25

julia> lcm(p,CycPol(q^3-1)) # lcm is fast between CycPols
(q-2)Φ₁Φ₂Φ₃Φ₂₃

julia> print(p)
CycPol(Pol([-2, 1]),0,(1,0),(2,0),(23,0)) # a format which can be read in Julia

Evaluating a CycPol at some Pol value gives in general a Pol. There are exceptions where we can keep the value a CycPol: evaluating at Pol()^n (that is q^n) or at Pol([E(n,k)],1) (that is qζₙᵏ). Then subs gives that evaluation:

julia> subs(p,Pol()^-1) # evaluate as a CycPol at q⁻¹
(2-q⁻¹)q⁻²⁴Φ₁Φ₂Φ₂₃

julia> using CyclotomicNumbers

julia> subs(p,Pol([E(2)],1)) # or at -q
(-q-2)Φ₁Φ₂Φ₄₆

The variable name used when printing a CycPol is the same as for Pols.

When showing a CycPol, some factors over extension fields of the cyclotomic polynomial Φₙ are given a special name. If n has a primitive root ξ, ϕ′ₙ is the product of the (q-ζ) where ζ runs over the odd powers of ξ, and ϕ″ₙ is the product for the even powers. Some further factors are recognized for small n.

julia> CycPol(q^6-E(4))
Φ″₈Φ⁽¹³⁾₂₄

The function show_factors gives the complete list of recognized factors for a given n:

julia> CycPols.show_factors(24)
15-element Vector{Tuple{CycPol{Int64}, Pol}}:
 (Φ₂₄, q⁸-q⁴+1)
 (Φ′₂₄, q⁴+ζ₃²)
 (Φ″₂₄, q⁴+ζ₃)
 (Φ‴₂₄, q⁴-√2q³+q²-√2q+1)
 (Φ⁗₂₄, q⁴+√2q³+q²+√2q+1)
 (Φ⁽⁵⁾₂₄, q⁴-√6q³+3q²-√6q+1)
 (Φ⁽⁶⁾₂₄, q⁴+√6q³+3q²+√6q+1)
 (Φ⁽⁷⁾₂₄, q⁴+√-2q³-q²-√-2q+1)
 (Φ⁽⁸⁾₂₄, q⁴-√-2q³-q²+√-2q+1)
 (Φ⁽⁹⁾₂₄, q²+ζ₃²√-2q-ζ₃)
 (Φ⁽¹⁰⁾₂₄, q²-ζ₃²√-2q-ζ₃)
 (Φ⁽¹¹⁾₂₄, q²+ζ₃√-2q-ζ₃²)
 (Φ⁽¹²⁾₂₄, q²-ζ₃√-2q-ζ₃²)
 (Φ⁽¹³⁾₂₄, q⁴-ζ₄q²-1)
 (Φ⁽¹⁴⁾₂₄, q⁴+ζ₄q²-1)

Such a factor can be obtained directly as:

julia> CycPol(;conductor=24,no=7)
Φ⁽⁷⁾₂₄

julia> CycPol(;conductor=24,no=7)(q)
Pol{Cyc{Int64}}: q⁴+√-2q³-q²-√-2q+1

This package also defines the function cylotomic_polynomial:

julia> p=cyclotomic_polynomial(24)
Pol{Int64}: q⁸-q⁴+1

julia> CycPol(p) # same as CycPol(;conductor=24,no=0)
Φ₂₄

source

# CycPols.subs — Function.

subs(p::CycPol,v::Pol)

a fast routine to compute CycPol(p(v)) but works for only two types of polynomials:

• v=Pol([e],1) for e a Root1, that is the value at qe for e=ζₙᵏ
• v=Pol([1],n) that is the value at qⁿ

julia> p=CycPol(Pol()^2-1)
Φ₁Φ₂

julia> subs(p,Pol([E(3)],1))
ζ₃²Φ″₃Φ′₆

julia> subs(p,Pol()^2)
Φ₁Φ₂Φ₄

source

# CycPols.cyclotomic_polynomial — Function.

cyclotomic_polynomial(n)

returns the n-th cyclotomic polynomial.

julia> cyclotomic_polynomial(5)
Pol{Int64}: q⁴+q³+q²+q+1

julia> cyclotomic_polynomial(24)
Pol{Int64}: q⁸-q⁴+1

source

# CycPols.CycPol — Type.

CycPols are internally a struct with fields:

.coeff: a coefficient, usually a Cyc or a Pol. The Pol case allows to represent as CycPols arbitrary Pols which is useful sometimes.

.valuation: an Int.

.v: a ModuleElt{Rational{Int},Int} where pairs r=>m give the multiplicity m of Root1(;r=r) as a root.

So CycPol(coeff,val,v) represents coeff*q^val*prod((q-Root1(;r=r))^m for (r,m) in v).

source