path: root/Math/Combinatorics/RootSystem.hs

-- Copyright (c) Yuchen Pei, 2017. 
-- Released under the GNU General Public License version 3 or later.
--
-- Copyright (c) David Amos, 2008-2015. All rights reserved.

module Math.Projects.RootSystem where

import Prelude hiding ( (*>) )

import Data.Ratio
import qualified Data.List as L
import qualified Data.Set as S

import Math.Algebra.LinearAlgebra
import Math.Algebra.Group.PermutationGroup hiding (elts, order, closure)
--import Math.Algebra.Group.SchreierSims as SS
--import Math.Algebra.Group.StringRewriting as SG

import Math.Algebra.Field.Base (Q)-- for Q

data Type = A | B | C | D | E | F | G deriving Show
type SimpleSystem = [[Q]]

-- Humphreys, Reflection Groups and Coxeter Groups


-- SIMPLE SYSTEMS
-- sometimes called fundamental systems

-- The ith basis vector in K^n
basisElt :: Int -> Int -> [Q] -- this type signature determines all the rest
basisElt n i = replicate (i-1) 0 ++ 1 : replicate (n-i) 0
-- We need to work over the rationals to ensure that arithmetic is exact
-- So long as our simple systems are rational, then reflection matrices are rational

-- A simple system is like a basis for the root system (see Humphreys p8 for full definition)
simpleSystem :: Type -> Int -> SimpleSystem
simpleSystem A n | n >= 1 = [e i <-> e (i+1) | i <- [1..n]]
    where e = basisElt (n+1)
simpleSystem B n | n >= 2 = [e i <-> e (i+1) | i <- [1..n-1]] ++ [e n]
    where e = basisElt n
simpleSystem C n | n >= 2 = [e i <-> e (i+1) | i <- [1..n-1]] ++ [2 *> e n]
    where e = basisElt n
simpleSystem D n | n >= 4 = [e i <-> e (i+1) | i <- [1..n-1]] ++ [e (n-1) <+> e n]
    where e = basisElt n
simpleSystem E n | n `elem` [6,7,8] = take n simpleroots
    where e = basisElt 8
          simpleroots = ((1/2) *> (e 1 <-> e 2 <-> e 3 <-> e 4 <-> e 5 <-> e 6 <-> e 7 <+> e 8))
                      : (e 1 <+> e 2)
                      : [e (i-1) <-> e (i-2) | i <- [3..8]]
simpleSystem F 4 = [e 2 <-> e 3, e 3 <-> e 4, e 4, (1/2) *> (e 1 <-> e 2 <-> e 3 <-> e 4)]
    where e = basisElt 4
simpleSystem G 2 = [e 1 <-> e 2, ((-2) *> e 1) <+> e 2 <+> e 3]
    where e = basisElt 3
simpleSystem t n = error $ "Invalid root system of type " ++ (show t) ++ " and rank " ++ (show n) ++ "."

-- ROOT SYSTEMS
-- Calculating the full root system from the fundamental roots

-- Humphreys p3
-- Weyl group element corresponding to a root
-- s alpha beta = s_\alpha \beta
s :: [Q] -> [Q] -> [Q]
s alpha beta = beta <-> (2 * (alpha <.> beta) / (alpha <.> alpha)) *> alpha

-- Given a simple system, return the full root system
-- The closure of a set of roots under reflection
{--allRoots :: SimpleSystem -> [[Q]]
allRoots ss = S.toList $ closure S.empty (S.fromList ss) where
    closure interior boundary
        | S.null boundary = interior
        | otherwise =
            let interior' = S.union interior boundary
                boundary' = S.fromList [s alpha beta | alpha <- ss, beta <- S.toList boundary] S.\\ interior'
            in closure interior' boundary'
            --}

positiveRoots :: SimpleSystem -> [[Q]]
positiveRoots ss = S.toList $ go S.empty (S.fromList ss) where
    go pr newPr
        | S.null newPr = pr
        | otherwise =
            let pr' = S.union pr newPr
                newPr' = S.fromList [s alpha (negate <$> beta) | alpha <- ss, beta <- S.toList newPr] S.\\ pr'
            in go pr' newPr'

{--
-- WEYL GROUP
-- The finite reflection group generated by the root system

-- Generators of the Weyl group as permutation group on the roots
weylPerms t n =
    let rs = simpleSystem t n
        xs = closure rs
        toPerm r = fromPairs [(x, w r x) | x <- xs]
    in map toPerm rs
    
-- Generators of the Weyl group as a matrix group
weylMatrices t n = map wMx (simpleSystem t n)

-- The Weyl group element corresponding to a root, represented as a matrix
wMx r = map (w r) [e i | i <- [1..d]] -- matrix for reflection in hyperplane orthogonal to r
    where d = length r -- dimension of the space
          e = basisElt d
-- the images of the basis elts form the columns of the matrix
-- however, reflection matrices are symmetric, so they also form the rows


-- CARTAN MATRIX, DYNKIN DIAGRAM, COXETER SYSTEM

cartanMatrix t n = [[2 * (ai <.> aj) / (ai <.> ai) | aj <- roots] | ai <- roots]
    where roots = simpleSystem t n
-- Note: The Cartan matrices for A, D, E systems are symmetric.
-- Those of B, C, F, G are not
-- Carter, Simple Groups of Lie Type, p44-5 gives the expected answers
-- They agree with our answers except for G2, which is the transpose
-- (So probably Carter defines the roots of G2 the other way round to Humphreys)

-- set the diagonal entries of (square) matrix mx to constant c
setDiag c mx@((x:xs):rs) = (c:xs) : zipWith (:) (map head rs) (setDiag c $ map tail rs)
setDiag _ [[]] = [[]]

-- Carter, Segal, Macdonald p17-18
-- given a Cartan matrix, derive the corresponding matrix describing the Dynkin diagram
-- nij = Aij * Aji, nii = 0
dynkinFromCartan aij = setDiag 0 $ (zipWith . zipWith) (*) aij (L.transpose aij)

dynkinDiagram t n = dynkinFromCartan $ cartanMatrix t n

-- given the Dynkin diagram nij, derive the coefficients mij of the Coxeter group <si | si^2, (sisj)^mij> (so mii == 1)
-- using nij = 4 cos^2 theta_ij
-- nij == 0 <=> theta = pi/2
-- nij == 1 <=> theta = pi/3
-- nij == 2 <=> theta = pi/4
-- nij == 3 <=> theta = pi/6
coxeterFromDynkin nij = setDiag 1 $ (map . map) f nij
    where f 0 = 2; f 1 = 3; f 2 = 4; f 3 = 6

-- The mij coefficients of the Coxeter group <si | si^2, (sisj)^mij>, as a matrix
coxeterMatrix t n = coxeterFromDynkin $ dynkinDiagram t n


-- Given the matrix of coefficients mij, return the Coxeter group <si | si^2, (sisj)^mij>
-- We assume but don't check that mii == 1 and mij == mji
fromCoxeterMatrix mx = (gs,rs) where
    n = length mx
    gs = map s_ [1..n]
    rs = rules mx 1
    rules [] _ = []
    rules ((1:xs):rs) i = ([s_ i, s_ i],[]) : [powerRelation i j m | (j,m) <- zip [i+1..] xs] ++ rules (map tail rs) (i+1)
    powerRelation i j m = (concat $ replicate m [s_ i, s_ j],[])

-- Another presentation for the Coxeter group, using braid relations
fromCoxeterMatrix2 mx = (gs,rs) where
    n = length mx
    gs = map s_ [1..n]
    rs = rules mx 1
    rules [] _ = []
    rules ((1:xs):rs) i = ([s_ i, s_ i],[]) : [braidRelation i j m | (j,m) <- zip [i+1..] xs] ++ rules (map tail rs) (i+1)
    braidRelation i j m = (take m $ cycle [s_ j, s_ i], take m $ cycle [s_ i, s_ j])



coxeterPresentation t n = fromCoxeterMatrix $ coxeterMatrix t n

eltsCoxeter t n = SG.elts $ fromCoxeterMatrix2 $ coxeterMatrix t n
-- it's just slightly faster to use the braid presentation

poincarePoly t n = map length $ L.group $ map length $ eltsCoxeter t n


-- LIE ALGEBRAS

elemMx n i j = replicate (i-1) z ++ e j : replicate (n-i) z
    where z = replicate n 0
          e = basisElt n


lieMult x y = x*y - y*x

-- for gluing matrices together
(+|+) = zipWith (++) -- glue two matrices together side by side
(+-+) = (++)         -- glue two matrices together above and below

form D n = (zMx n +|+ idMx n)
                  +-+
          (idMx n +|+ zMx n)
form C n = (2 : replicate (2*n) 0) :
           (map (0:) (form D n))
form B n = let id' = (-1) *>> idMx n
           in (zMx n +|+ idMx n)
                     +-+
               (id'  +|+ zMx n)


-- TESTING
-- The expected values of the root system, number of roots, order of Weyl group
-- for comparison against the calculated values

-- !! Not yet got root systems for E6,7,8, F4

-- Humphreys p41ff

-- The full root system
-- L.sort (rootSystem t n) == closure (simpleSystem t n)
-- rootSystem :: Type -> Int -> [[QQ]]
rootSystem A n | n >= 1 = [e i <-> e j | i <- [1..n+1], j <- [1..n+1], i /= j]
    where e = basisElt (n+1)
rootSystem B n | n >= 2 = shortRoots ++ longRoots
    where e = basisElt n
          shortRoots = [e i | i <- [1..n]]
                    ++ [[] <-> e i | i <- [1..n]]
          longRoots  = [e i <+> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [e i <-> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [[] <-> e i <+> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [[] <-> e i <-> e j | i <- [1..n], j <- [i+1..n]]
rootSystem C n | n >= 2 = longRoots ++ shortRoots
    where e = basisElt n
          longRoots  = [2 *> e i | i <- [1..n]]
                    ++ [[] <-> (2 *> e i) | i <- [1..n]]
          shortRoots = [e i <+> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [e i <-> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [[] <-> e i <+> e j | i <- [1..n], j <- [i+1..n]]
                    ++ [[] <-> e i <-> e j | i <- [1..n], j <- [i+1..n]]
rootSystem D n | n >= 4 =
    [e i <+> e j | i <- [1..n], j <- [i+1..n]]
    ++ [e i <-> e j | i <- [1..n], j <- [i+1..n]]
    ++ [[] <-> e i <+> e j | i <- [1..n], j <- [i+1..n]]
    ++ [[] <-> e i <-> e j | i <- [1..n], j <- [i+1..n]]
    where e = basisElt n
rootSystem G 2 = shortRoots ++ longRoots
    where e = basisElt 3
          shortRoots = [e i <-> e j | i <- [1..3], j <- [1..3], i /= j]
          longRoots = concatMap (\r-> [r,[] <-> r]) [2 *> e i <-> e j <-> e k | i <- [1..3], [j,k] <- [[1..3] L.\\ [i]] ]


-- numRoots t n == length (closure $ simpleSystem t n)
numRoots A n = n*(n+1)
numRoots B n = 2*n*n
numRoots C n = 2*n*n
numRoots D n = 2*n*(n-1)
numRoots E 6 = 72
numRoots E 7 = 126
numRoots E 8 = 240    
numRoots F 4 = 48
numRoots G 2 = 12

-- The order of the Weyl group
-- orderWeyl t n == S.order (weylPerms t n)
orderWeyl A n = factorial (n+1)
orderWeyl B n = 2^n * factorial n
orderWeyl C n = 2^n * factorial n
orderWeyl D n = 2^(n-1) * factorial n
orderWeyl E 6 = 2^7 * 3^4 * 5
orderWeyl E 7 = 2^10 * 3^4 * 5 * 7
orderWeyl E 8 = 2^14 * 3^5 * 5^2 * 7
orderWeyl F 4 = 2^7 * 3^2
orderWeyl G 2 = 12


factorial n = product [1..toInteger n]


{-
-- now moved to TRootSystem
test1 = all (\(t,n) -> orderWeyl t n == L.genericLength (eltsCoxeter t n))
    [(A,3),(A,4),(A,5),(B,3),(B,4),(B,5),(C,3),(C,4),(C,5),(D,4),(D,5),(F,4),(G,2)]

test2 = all (\(t,n) -> orderWeyl t n == SS.order (weylPerms t n))
    [(A,3),(A,4),(A,5),(B,3),(B,4),(B,5),(C,3),(C,4),(C,5),(D,4),(D,5),(E,6),(F,4),(G,2)]
-}
--}