-- Copyright (c) David Amos, 2008. All rights reserved. module Math.Algebra.Group.SchreierSims where import qualified Data.List as L import Data.Maybe (isNothing, isJust) import qualified Data.Map as M import Math.Algebra.Group.PermutationGroup hiding (elts, order, gens, isMember, isSubgp, isNormal, reduceGens, normalClosure, commutatorGp, derivedSubgp) -- COSET REPRESENTATIVES FOR STABILISER OF A POINT -- Given a group G = <gs>, and a point x, find coset representatives for Gx (stabiliser of x) in G -- In other words, for each x' in the orbit of x under G, we find a g <- G taking x to x' -- The code is similar to the code for calculating orbits, but modified to keep track of the group elements that we used to get there cosetRepsGx gs x = cosetRepsGx' gs M.empty (M.singleton x 1) where cosetRepsGx' gs interior boundary | M.null boundary = interior | otherwise = let interior' = M.union interior boundary boundary' = M.fromList [(p .^ g, h*g) | g <- gs, (p,h) <- M.toList boundary] M.\\ interior' in cosetRepsGx' gs interior' boundary' -- SCHREIER GENERATORS -- toSet xs = (map head . group . sort) xs -- Generators for Gx, the stabiliser of x, given that G is generated by gs, and rs is a set of coset representatives for Gx in G. -- Schreier's Lemma states that if H < G = <S>, and R is a set of coset reps for H in G -- then H is generated by { rs(rs)*^-1 | r <- R, s <- S } (where * means "the coset representative of") -- In particular, with H = Gx, this gives us a way of finding a set of generators for Gx schreierGeneratorsGx (x,rs) gs = L.nub $ filter (/= 1) [schreierGenerator r g | r <- M.elems rs, g <- gs] where schreierGenerator r g = let h = r * g h' = rs M.! (x .^ h) in h * inverse h' -- SCHREIER-SIMS ALGORITHM sift bts g = sift' bts g where sift' [] g = if g == 1 then Nothing else Just g sift' ((b,t):bts) g = case M.lookup (b .^ g) t of Nothing -> Just g Just h -> sift' bts (g * inverse h) findBase gs = minimum $ concatMap supp gs {- -- Find base and strong generating set using Schreier-Sims algorithm bsgs gs | all (/= 1) gs = map fst $ ss [newLevel gs] [] newLevel s = let b = findBase s t = cosetRepsGx s b in ((b,t),s) ss (bad@((b,t),s):bads) goods = let bts = map fst goods sgs = schreierGeneratorsGx (b,t) s siftees = filter isJust $ map (sift bts) sgs in if null siftees then ss bads (bad:goods) else let Just h = head siftees in if null goods then ss (newLevel [h] : bad : bads) [] else let ((b_,t_),s_) = head goods s' = h:s_ t' = cosetRepsGx s' b_ in ss (((b_,t'),s') : bad : bads) (tail goods) ss [] goods = goods -} -- Find base and strong generating set using Schreier-Sims algorithm -- This version guarantees to use bases in order bsgs gs = bsgs' bs gs where bs = (map head . L.group . L.sort) $ concatMap supp gs -- This version lets you pass in bases in the order you want them (or [], and it will find its own) bsgs' bs gs = map fst $ ss bs gs -- For example, bsgs (_A 5) uses [1,2,3] as the bases, but bsgs' [] (_A 5) uses [1,3,2] newLevel (b:bs) s = (bs, newLevel' b s) newLevel [] s = ([], newLevel' b s) where b = findBase s newLevel' b s = ((b,t),s) where t = cosetRepsGx s b ss bs gs = ss' bs' [level] [] where (bs',level) = newLevel bs gs ss' bs (bad@((b,t),s):bads) goods = let bts = map fst goods sgs = schreierGeneratorsGx (b,t) s siftees = filter isJust $ map (sift bts) sgs in if null siftees then ss' bs bads (bad:goods) else let Just h = head siftees in if null goods then let (bs', level) = newLevel bs [h] in ss' bs' (level : bad : bads) [] else let ((b_,t_),s_) = head goods s' = h:s_ t' = cosetRepsGx s' b_ in ss' bs (((b_,t'),s') : bad : bads) (tail goods) ss' _ [] goods = goods {- extendbsgs [] g = bsgs [g] extendbsgs (((b,t),s):bts) g = ss (((b,t),g:s):bts) [] bsgs' gs = map fst $ foldl extendbsgs [] gs -} -- The above is written for simplicity. -- Its efficiency could be improved by incrementally updating the transversals, -- and keeping track of Schreier generators we have already tried. -- (Remember to add new Schreier generators every time the generating set or transversal is augmented.) -- USING THE SCHREIER-SIMS TRANSVERSALS isMemberBSGS bts g = isNothing $ sift bts g -- By Lagrange's thm, every g <- G can be written uniquely as g = r_m ... r_1 (Seress p56) -- Note that we have to reverse the list of coset representatives eltsBSGS bts = map (product . reverse) (cartProd ts) where ts = map (M.elems . snd) bts cartProd (set:sets) = [x:xs | x <- set, xs <- cartProd sets] cartProd [] = [[]] orderBSGS bts = product (map (toInteger . M.size . snd) bts) isMember gs h = isMemberBSGS (bsgs gs) h elts gs = eltsBSGS $ bsgs gs order [] = 1 order gs = orderBSGS $ bsgs gs isSubgp hs gs = all (isMemberBSGS gs') hs where gs' = bsgs gs isNormal hs gs = hs `isSubgp` gs && all (isMemberBSGS hs') [h~^g | h <- hs, g <- gs] where hs' = bsgs hs index gs hs = order gs `div` order hs -- strong generating set -- sgs gs = filter (/=1) $ concatMap (M.elems . snd) $ bsgs gs -- sgs gs = concatMap snd $ ss [newLevel gs] [] sgs gs = L.nub $ concatMap snd $ ss bs gs -- bs' [level] [] where bs = (map head . L.group . L.sort) $ concatMap supp gs -- (bs',level) = newLevel bs gs -- !! Note, not properly tested - results not in expected order -- the sgs calculated during bsgs may be a smaller set (for example, the whole transversal could be powers of a single generator). -- given list of generators, try to find a shorter list reduceGens gs = fst $ reduceGensBSGS (filter (/=1) gs) reduceGensBSGS (g:gs) = reduceGens' ([g],bsgs [g]) gs where reduceGens' (gs,bsgsgs) (h:hs) = if isMemberBSGS bsgsgs h then reduceGens' (gs,bsgsgs) hs else reduceGens' (h:gs, bsgs $ h:gs) hs reduceGens' (gs,bsgsgs) [] = (reverse gs,bsgsgs) reduceGensBSGS [] = ([],[]) -- normal closure of H in G -- for efficiency, should be called with gs and hs already reduced sets of generators normalClosure gs hs = reduceGens $ hs ++ [h ~^ g | h <- hs, g <- gs'] where gs' = gs ++ map inverse gs -- commutator gp of H and K commutatorGp hs ks = normalClosure (hsks) [h^-1 * k^-1 * h * k | h <- hs', k <- ks'] where hs' = reduceGens hs ks' = reduceGens ks hsks = reduceGens (hs' ++ ks') -- no point processing more potential generators than we have to -- derived subgroup (or commutator subgroup) derivedSubgp gs = normalClosure gs' [g^-1 * h^-1 * g * h | g <- gs', h <- gs'] where gs' = reduceGens gs -- == commutatorGp gs gs {- isPerfect gs = order gs == order (derivedSubgp gs) -- We compare orders rather than the generators themselves, because derivedSubgp will usually find different generators derivedSeries gs = derivedSeries' (gs, order gs) where derivedSeries' ([],1) = [[]] derivedSeries' (hs, orderhs) = let hs' = derivedSubgp hs orderhs' = order hs' in if orderhs' == orderhs then [hs] else hs : derivedSeries' (hs',orderhs') lowerCentralSeries gs = lowerCentralSeries' (gs, order gs) where lowerCentralSeries' ([],1) = [[]] lowerCentralSeries' (hs, orderhs) = let hs' = commutatorGp gs hs orderhs' = order hs' in if orderhs' == orderhs then [hs] else hs : lowerCentralSeries' (hs',orderhs') -}