-- Copyright (c) David Amos, 2009. All rights reserved. module Math.Combinatorics.GraphAuts where import qualified Data.List as L import qualified Data.Map as M import qualified Data.Set as S import Math.Common.ListSet import Math.Combinatorics.Graph -- import Math.Combinatorics.StronglyRegularGraph -- import Math.Combinatorics.Hypergraph -- can't import this, creates circular dependency import Math.Algebra.Group.PermutationGroup import Math.Algebra.Group.SchreierSims as SS -- The code for finding automorphisms - "graphAuts" - follows later on in file -- TRANSITIVITY PROPERTIES OF GRAPHS isVertexTransitive (G [] []) = True -- null graph is trivially vertex transitive isVertexTransitive g@(G (v:vs) es) = orbitP auts v == v:vs where auts = graphAuts g isEdgeTransitive (G _ []) = True isEdgeTransitive g@(G vs (e:es)) = orbitB auts e == e:es where auts = graphAuts g arc ->^ g = map (.^ g) arc -- unlike blocks, arcs are directed, so the action on them does not sort -- Godsil & Royle 59-60 isArcTransitive (G _ []) = True -- empty graphs are trivially arc transitive isArcTransitive g@(G vs es) = orbit (->^) a auts == a:as where a:as = L.sort $ es ++ map reverse es auts = graphAuts g isArcTransitive' g@(G (v:vs) es) = orbitP auts v == v:vs && -- isVertexTransitive g orbitP stab n == n:ns where auts = graphAuts g stab = dropWhile (\p -> v .^ p /= v) auts -- we know that graphAuts are returned in this order n:ns = nbrs g v -- execution time of both of the above is dominated by the time to calculate the graph auts, so their performance is similar -- then k n, kb n n, q n, other platonic solids, petersen graph, heawood graph, pappus graph, desargues graph are all arc-transitive -- find arcs of length l from x using dfs - results returned in order -- an arc is a sequence of vertices connected by edges, no doubling back, but self-crossings allowed findArcs g@(G vs es) x l = map reverse $ dfs [ ([x],0) ] where dfs ( (z1:z2:zs,l') : nodes) | l == l' = (z1:z2:zs) : dfs nodes | otherwise = dfs $ [(w:z1:z2:zs,l'+1) | w <- nbrs g z1, w /= z2] ++ nodes dfs ( ([z],l') : nodes) | l == l' = [z] : dfs nodes | otherwise = dfs $ [([w,z],l'+1) | w <- nbrs g z] ++ nodes dfs [] = [] -- note that a graph with triangles can't be 3-arc transitive, etc, because an aut can't map a self-crossing arc to a non-self-crossing arc isnArcTransitive _ (G [] []) = True isnArcTransitive n g@(G (v:vs) es) = orbitP auts v == v:vs && -- isVertexTransitive g orbit (->^) a stab == a:as where auts = graphAuts g stab = dropWhile (\p -> v .^ p /= v) auts -- we know that graphAuts are returned in this order a:as = findArcs g v n is2ArcTransitive g = isnArcTransitive 2 g is3ArcTransitive g = isnArcTransitive 3 g -- Godsil & Royle 66-7 isDistanceTransitive (G [] []) = True isDistanceTransitive g@(G (v:vs) es) | isConnected g = orbitP auts v == v:vs && -- isVertexTransitive g length stabOrbits == diameter g + 1 -- the orbits under the stabiliser of v coincide with the distance partition from v | otherwise = error "isDistanceTransitive: only defined for connected graphs" where auts = graphAuts g stab = dropWhile (\p -> v .^ p /= v) auts -- we know that graphAuts are returned in this order stabOrbits = let os = orbits stab in os ++ map (:[]) ((v:vs) L.\\ concat os) -- include fixed point orbits -- GRAPH AUTOMORPHISMS -- refine one partition by another refine p1 p2 = concat [ [c1 `intersect` c2 | c2 <- p2] | c1 <- p1] -- Refinement preserves ordering within cells but not between cells -- eg the cell [1,2,3,4] could be refined to [2,4],[1,3] isGraphAut (G vs es) h = all (`S.member` es') [e -^ h | e <- es] where es' = S.fromList es -- this works best on sparse graphs, where p(edge) < 1/2 -- if p(edge) > 1/2, it would be better to test on the complement of the graph -- ALTERNATIVE VERSIONS OF GRAPH AUTS -- (showing how we got to the final version) -- return all graph automorphisms, using naive depth first search graphAuts1 (G vs es) = dfs [] vs vs where dfs xys (x:xs) ys = concat [dfs ((x,y):xys) xs (L.delete y ys) | y <- ys, isCompatible (x,y) xys] dfs xys [] [] = [fromPairs xys] isCompatible (x,y) xys = and [([x',x] `S.member` es') == (L.sort [y,y'] `S.member` es') | (x',y') <- xys] es' = S.fromList es -- return generators for graph automorphisms -- (using Lemma 9.1.1 from Seress p203 to prune the search tree) graphAuts2 (G vs es) = graphAuts' [] vs where graphAuts' us (v:vs) = let uus = zip us us in concat [take 1 $ dfs ((v,w):uus) vs (v : L.delete w vs) | w <- vs, isCompatible (v,w) uus] ++ graphAuts' (v:us) vs -- stab us == transversal for stab (v:us) ++ stab (v:us) (generators thereof) graphAuts' _ [] = [] -- we're not interested in finding the identity element dfs xys (x:xs) ys = concat [dfs ((x,y):xys) xs (L.delete y ys) | y <- ys, isCompatible (x,y) xys] dfs xys [] [] = [fromPairs xys] isCompatible (x,y) xys = and [([x',x] `S.member` es') == (L.sort [y,y'] `S.member` es') | (x',y') <- xys] es' = S.fromList es -- Now using distance partitions graphAuts3 g@(G vs es) = graphAuts' [] [vs] where graphAuts' us ((x:ys):pt) = let px = refine (ys : pt) (dps M.! x) p y = refine ((x : L.delete y ys) : pt) (dps M.! y) uus = zip us us p' = L.sort $ filter (not . null) $ px in concat [take 1 $ dfs ((x,y):uus) px (p y) | y <- ys] ++ graphAuts' (x:us) p' graphAuts' us ([]:pt) = graphAuts' us pt graphAuts' _ [] = [] dfs xys p1 p2 | map length p1 /= map length p2 = [] | otherwise = let p1' = filter (not . null) p1 p2' = filter (not . null) p2 in if all isSingleton p1' then let xys' = xys ++ zip (concat p1') (concat p2') in if isCompatible xys' then [fromPairs' xys'] else [] else let (x:xs):p1'' = p1' ys:p2'' = p2' in concat [dfs ((x,y):xys) (refine (xs : p1'') (dps M.! x)) (refine ((L.delete y ys):p2'') (dps M.! y)) | y <- ys] isCompatible xys = and [([x,x'] `S.member` es') == (L.sort [y,y'] `S.member` es') | (x,y) <- xys, (x',y') <- xys, x < x'] dps = M.fromList [(v, distancePartition g v) | v <- vs] es' = S.fromList es isSingleton [_] = True isSingleton _ = False -- Now we try to use generators we've already found at a given level to save us having to look for others -- For example, if we have found (1 2)(3 4) and (1 3 2), then we don't need to look for something taking 1 -> 4 graphAuts g@(G vs es) = graphAuts' [] [vs] where graphAuts' us p@((x:ys):pt) = let p' = L.sort $ filter (not . null) $ refine (ys:pt) (dps M.! x) in level us p x ys [] ++ graphAuts' (x:us) p' graphAuts' us ([]:pt) = graphAuts' us pt graphAuts' _ [] = [] level us p@(ph:pt) x (y:ys) hs = let px = refine (L.delete x ph : pt) (dps M.! x) py = refine (L.delete y ph : pt) (dps M.! y) uus = zip us us in case dfs ((x,y):uus) px py of [] -> level us p x ys hs h:_ -> let hs' = h:hs in h : level us p x (ys L.\\ (x .^^ hs')) hs' level _ _ _ [] _ = [] dfs xys p1 p2 | map length p1 /= map length p2 = [] | otherwise = let p1' = filter (not . null) p1 p2' = filter (not . null) p2 in if all isSingleton p1' then let xys' = xys ++ zip (concat p1') (concat p2') in if isCompatible xys' then [fromPairs' xys'] else [] else let (x:xs):p1'' = p1' ys:p2'' = p2' in concat [dfs ((x,y):xys) (refine (xs : p1'') (dps M.! x)) (refine ((L.delete y ys):p2'') (dps M.! y)) | y <- ys] isCompatible xys = and [([x,x'] `S.member` es') == (L.sort [y,y'] `S.member` es') | (x,y) <- xys, (x',y') <- xys, x < x'] dps = M.fromList [(v, distancePartition g v) | v <- vs] es' = S.fromList es -- contrary to first thought, you can't stop when a level is null - eg kb 2 3, the third level is null, but the fourth isn't removeGens x gs = removeGens' [] gs where baseOrbit = x .^^ gs removeGens' ls (r:rs) = if x .^^ (ls++rs) == baseOrbit then removeGens' ls rs else removeGens' (r:ls) rs removeGens' ls [] = reverse ls -- !! reverse is probably pointless -- !! DON'T THINK THIS IS WORKING PROPERLY -- eg graphAutsSGSNew $ toGraph ([1..7],[[1,3],[2,3],[3,4],[4,5],[4,6],[4,7]]) -- returns [[[1,2]],[[5,6]],[[5,7,6]],[[6,7]]] -- whereas [[6,7]] was a Schreier generator, so shouldn't have been listed -- Using Schreier generators to seed the next level -- At the moment this is slower than the above -- (This could be modified to allow us to start the search with a known subgroup) graphAutsNew g@(G vs es) = graphAuts' [] [] [vs] where graphAuts' us hs p@((x:ys):pt) = let ys' = ys L.\\ (x .^^ hs) -- don't need to consider points which can already be reached from Schreier generators hs' = level us p x ys' [] p' = L.sort $ filter (not . null) $ refine (ys:pt) (dps M.! x) reps = cosetRepsGx (hs'++hs) x schreierGens = removeGens x $ schreierGeneratorsGx (x,reps) (hs'++hs) in hs' ++ graphAuts' (x:us) schreierGens p' graphAuts' us hs ([]:pt) = graphAuts' us hs pt graphAuts' _ _ [] = [] level us p@(ph:pt) x (y:ys) hs = let px = refine (L.delete x ph : pt) (dps M.! x) py = refine (L.delete y ph : pt) (dps M.! y) uus = zip us us in if map length px /= map length py then level us p x ys hs else case dfs ((x,y):uus) (filter (not . null) px) (filter (not . null) py) of [] -> level us p x ys hs h:_ -> let hs' = h:hs in h : level us p x (ys L.\\ (x .^^ hs')) hs' -- if h1 = (1 2)(3 4), and h2 = (1 3 2), then we can remove 4 too level _ _ _ [] _ = [] dfs xys p1 p2 | map length p1 /= map length p2 = [] | otherwise = let p1' = filter (not . null) p1 p2' = filter (not . null) p2 in if all isSingleton p1' then let xys' = xys ++ zip (concat p1') (concat p2') in if isCompatible xys' then [fromPairs' xys'] else [] else let (x:xs):p1'' = p1' ys:p2'' = p2' in concat [dfs ((x,y):xys) (refine (xs : p1'') (dps M.! x)) (refine ((L.delete y ys):p2'') (dps M.! y)) | y <- ys] isCompatible xys = and [([x,x'] `S.member` es') == (L.sort [y,y'] `S.member` es') | (x,y) <- xys, (x',y') <- xys, x < x'] dps = M.fromList [(v, distancePartition g v) | v <- vs] es' = S.fromList es -- GRAPH ISOMORPHISMS graphIsos g1 g2 = concat [dfs [] (distancePartition g1 v1) (distancePartition g2 v2) | v2 <- vertices g2] where v1 = head $ vertices g1 dfs xys p1 p2 | map length p1 /= map length p2 = [] | otherwise = let p1' = filter (not . null) p1 p2' = filter (not . null) p2 in if all isSingleton p1' then let xys' = xys ++ zip (concat p1') (concat p2') in if isCompatible xys' then [xys'] else [] else let (x:xs):p1'' = p1' ys:p2'' = p2' in concat [dfs ((x,y):xys) (refine (xs : p1'') (dps1 M.! x)) (refine ((L.delete y ys):p2'') (dps2 M.! y)) | y <- ys] isCompatible xys = and [([x,x'] `S.member` es1) == (L.sort [y,y'] `S.member` es2) | (x,y) <- xys, (x',y') <- xys, x < x'] dps1 = M.fromList [(v, distancePartition g1 v) | v <- vertices g1] dps2 = M.fromList [(v, distancePartition g2 v) | v <- vertices g2] es1 = S.fromList $ edges g1 es2 = S.fromList $ edges g2 isIso g1 g2 = (not . null) (graphIsos g1 g2) -- graphAuts3 g = map fromPairs $ graphIsos g g {- graphAuts2 (G vs es) = graphAuts' [] 1 (bsgsSym vs) where graphAuts' bs g ((b,t):bts) = concat [graphAuts' (b:bs) (h*g) bts | h <- M.elems t, isCompatible (b:bs) (h*g)] -- has to be h*g not g*h - not quite sure why graphAuts' _ g [] = [g] isCompatible (b:bs) g = and [(e `S.member` es') == ((e -^ g) `S.member` es') | e <- [ [b',b] | b' <- bs] ] -- if bs ordered then b' < b es' = S.fromList es graphAutsSGS2 (G vs es) = transversals [] (bsgsSym vs) where transversals bs ((b,t):bts) = let t' = concat [take 1 $ dfs (b:bs) h bts | h <- tail (M.elems t), isCompatible (b:bs) h] in t' ++ transversals (b:bs) bts transversals _ [] = [] dfs bs g ((b,t):bts) = concat [dfs (b:bs) (h*g) bts | h <- M.elems t, isCompatible (b:bs) (h*g)] dfs _ g [] = [g] isCompatible (b:bs) g = and [(e `S.member` es') == ((e -^ g) `S.member` es') | e <- [ [b',b] | b' <- bs] ] -- if bs ordered then b' < b es' = S.fromList es -- base and strong generating set for Sym(xs) bsgsSym xs = [(x, t x) | x <- init xs] where t x = M.fromList $ (x,p []) : [(y, p [[x,y]]) | y <- dropWhile (<= x) xs] bsgs_S n = bsgsSym [1..n] -}