-- Copyright (c) David Amos, 2008. All rights reserved. -- |A module providing a type for non-commutative polynomials. module Math.Algebra.NonCommutative.NCPoly where import Data.List as L import Math.Algebra.Field.Base -- (NON-COMMUTATIVE) MONOMIALS newtype Monomial v = M [v] deriving (Eq) instance Ord v => Ord (Monomial v) where compare (M xs) (M ys) = compare (length xs,xs) (length ys,ys) -- Glex ordering instance (Eq v, Show v) => Show (Monomial v) where show (M xs) | null xs = "1" | otherwise = concatMap showPower (L.group xs) where showPower [v] = showVar v showPower vs@(v:_) = showVar v ++ "^" ++ show (length vs) showVar v = filter (/= '"') (show v) -- Taken from NonComMonomial - why don't we just use it directly instance (Eq v, Show v) => Num (Monomial v) where M xs * M ys = M (xs ++ ys) fromInteger 1 = M [] -- try to find l, r such that a = lbr divM (M a) (M b) = divM' [] a where divM' ls (r:rs) = if b `L.isPrefixOf` (r:rs) then Just (M $ reverse ls, M $ drop (length b) (r:rs)) else divM' (r:ls) rs divM' _ [] = Nothing -- (NON-COMMUTATIVE) POLYNOMIALS newtype NPoly r v = NP [(Monomial v,r)] deriving (Eq) instance (Ord r, Ord v) => Ord (NPoly r v) where compare (NP ts) (NP us) = compare ts us instance (Show r, Eq v, Show v) => Show (NPoly r v) where show (NP []) = "0" show (NP ts) = let (c:cs) = concatMap showTerm ts in if c == '+' then cs else c:cs where showTerm (m,a) = case show a of "1" -> "+" ++ show m "-1" -> "-" ++ show m -- cs@(x:_) -> (if x == '-' then cs else '+':cs) ++ (if m == 1 then "" else show m) cs -> showCoeff cs ++ (if m == 1 then "" else show m) showCoeff (c:cs) = if any (`elem` ['+','-']) cs then "+(" ++ c:cs ++ ")" else if c == '-' then c:cs else '+':c:cs instance (Eq r, Num r, Ord v, Show v) => Num (NPoly r v) where NP ts + NP us = NP (mergeTerms ts us) negate (NP ts) = NP $ map (\(m,c) -> (m,-c)) ts NP ts * NP us = NP $ collect $ L.sortBy cmpTerm $ [(g*h,c*d) | (g,c) <- ts, (h,d) <- us] fromInteger 0 = NP [] fromInteger n = NP [(fromInteger 1, fromInteger n)] cmpTerm (a,c) (b,d) = case compare a b of EQ -> EQ; GT -> LT; LT -> GT -- in mpolys we put "larger" terms first -- inputs in descending order mergeTerms (t@(g,c):ts) (u@(h,d):us) = case cmpTerm t u of LT -> t : mergeTerms ts (u:us) GT -> u : mergeTerms (t:ts) us EQ -> if e == 0 then mergeTerms ts us else (g,e) : mergeTerms ts us where e = c + d mergeTerms ts us = ts ++ us -- one of them is null collect (t1@(g,c):t2@(h,d):ts) | g == h = collect $ (g,c+d):ts | c == 0 = collect $ t2:ts | otherwise = t1 : collect (t2:ts) collect ts = ts -- Fractional instance so that we can enter fractional coefficients -- Only lets us divide by field elements (with unit monomial), not any other polynomials instance (Eq k, Fractional k, Ord v, Show v) => Fractional (NPoly k v) where recip (NP [(1,c)]) = NP [(1, recip c)] recip _ = error "NPoly.recip: only supported for (non-zero) constants" -- SOME VARIABLES (INDETERMINATES) -- The idea is that you define your own type of indeterminates as required, along the same lines as this data Var = X | Y | Z deriving (Eq,Ord) instance Show Var where show X = "x" show Y = "y" show Z = "z" -- |Create a non-commutative variable for use in forming non-commutative polynomials. -- For example, we could define x = var "x", y = var "y". Then x*y /= y*x. var :: (Num k) => v -> NPoly k v var v = NP [(M [v], 1)] x = var X :: NPoly Q Var y = var Y :: NPoly Q Var z = var Z :: NPoly Q Var -- DIVISION ALGORITHM lm (NP ((m,c):ts)) = m lc (NP ((m,c):ts)) = c lt (NP (t:ts)) = NP [t] -- given f, gs, find ls, rs, f' such that f = sum (zipWith3 (*) ls gs rs) + f', with f' not divisible by any g quotRemNP f gs | all (/=0) gs = quotRemNP' f (replicate n (0,0), 0) | otherwise = error "quotRemNP: division by zero" where n = length gs quotRemNP' 0 (lrs,f') = (lrs,f') quotRemNP' h (lrs,f') = divisionStep h (gs,[],lrs,f') divisionStep h (g:gs, lrs', (l,r):lrs, f') = case lm h `divM` lm g of Just (l',r') -> let l'' = NP [(l',lc h / lc g)] r'' = NP [(r',1)] h' = h - l'' * g * r'' in quotRemNP' h' (reverse lrs' ++ (l+l'',r+r''):lrs, f') Nothing -> divisionStep h (gs,(l,r):lrs',lrs,f') divisionStep h ([],lrs',[],f') = let lth = lt h -- can't reduce lt h, so add it to the remainder and try to reduce the remaining terms in quotRemNP' (h-lth) (reverse lrs', f'+lth) -- It is only marginally (5-10%) more space/time efficient not to track the (lazily unevaluated) factors remNP f gs | all (/=0) gs = remNP' f 0 -- let result = remNP' f 0 in if result == remNP2 f gs then result else error ("remNP2 " ++ show f ++ " " ++ show gs) | otherwise = error "remNP: division by zero" where n = length gs remNP' 0 f' = f' remNP' h f' = divisionStep h gs f' divisionStep h (g:gs) f' = case lm h `divM` lm g of Just (l',r') -> let l'' = NP [(l',lc h / lc g)] r'' = NP [(r',1)] h' = h - l'' * g * r'' in remNP' h' f' Nothing -> divisionStep h gs f' divisionStep h [] f' = let lth = lt h -- can't reduce lt h, so add it to the remainder and try to reduce the remaining terms in remNP' (h-lth) (f'+lth) infixl 7 %% -- f %% gs = r where (_,r) = quotRemNP f gs f %% gs = remNP f gs -- !! Not sure if the following is valid -- The idea is to avoid dividing by lc g, because sometimes our coefficient ring is not a field -- Passes all the knot theory tests -- However, it may be that if we ever get a non-invertible element at the front, we are in trouble anyway remNP2 f gs | all (/=0) gs = remNP' f 0 | otherwise = error "remNP: division by zero" where n = length gs remNP' 0 f' = f' remNP' h f' = divisionStep h gs f' divisionStep h (g:gs) f' = case lm h `divM` lm g of Just (l',r') -> let l'' = NP [(l',1)] -- NP [(l',lc h / lc g)] r'' = NP [(r',1)] lcg = inject (lc g) lch = inject (lc h) -- h' = h - l'' * g * r'' h' = lcg * h - lch * l'' * g * r'' in remNP' h' (lcg * f') -- must multiply f' by lcg too (otherwise get incorrect results, eg tlBasis 4) Nothing -> divisionStep h gs f' divisionStep h [] f' = let lth = lt h -- can't reduce lt h, so add it to the remainder and try to reduce the remaining terms in remNP' (h-lth) (f'+lth) -- OTHER STUFF toMonic 0 = 0 toMonic (NP ts@((_,c):_)) | c == 1 = NP ts | otherwise = NP $ map (\(m,d)->(m,d/c)) ts -- injection of field elements into polynomial ring inject 0 = NP [] inject c = NP [(fromInteger 1, c)] -- substitute terms for variables in an NPoly -- eg subst [(x,a),(y,a+b),(z,c^2)] (x*y+z) -> a*(a+b)+c^2 subst vts (NP us) = sum [inject c * substM m | (m,c) <- us] where substM (M xs) = product [substV x | x <- xs] substV v = let v' = NP [(M [v], 1)] in case L.lookup v' vts of Just t -> t Nothing -> error ("subst: no substitute supplied for " ++ show v') -- INVERTIBLE -- To support algebras which have invertible elements class Invertible a where inv :: a -> a x ^- k = inv x ^ k