This is the smallest representable rank 3 matroid which is divisionally free but not inductively free. It has 14 atoms (cf. [BBJ+21]).
gap> #db := AttachMatroidsDatabase( ); # needs arangosh gap> #key := "ef53049d834fba1b21f36c365d7f1d46d7fce2f2"; gap> #d := db.matroids_split_public.document( key ); gap> d := [ [ 1, 2, 3, 4, 5 ], [ 1, 6, 7, 8, 9 ], > [ 1, 10, 11, 12 ], [ 2, 6, 10, 13 ], [ 2, 7, 11, 14 ], > [ 3, 6, 12, 14 ], [ 3, 8, 11, 13 ], [ 4, 9, 10, 14 ], > [ 4, 7, 13 ], [ 5, 7, 12 ], [ 5, 8, 10 ], > [ 5, 9, 11 ], [ 5, 13, 14 ], [ 9, 12, 13 ], > [ 1, 13 ], [ 1, 14 ], [ 2, 8 ], [ 2, 9 ], [ 2, 12 ], [ 3, 7 ], > [ 3, 9 ], [ 3, 10 ], [ 4, 6 ], [ 4, 8 ], [ 4, 11 ], [ 4, 12 ], > [ 5, 6 ], [ 6, 11 ], [ 7, 10 ], [ 8, 12 ], [ 8, 14 ] ];; gap> matroid := MatroidByCoatomsNC( 14, 3, d ); <A matroid> gap> #homalgIOMode( "f" ); gap> zz := HomalgRingOfIntegersInSingular( ); Z gap> SetInfoLevel( InfoWarning, 0 ); gap> M := EquationsAndInequationsOfModuliSpaceOfMatroid( matroid, zz );; gap> LoadPackage( "ZariskiFrames", ">= 2023.06-06" ); true gap> m := DistinguishedQuasiAffineSet( M ); V_{Z[a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12]}( I ) \ V_{Z[a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12]}( J_1 ) \ .. \ V_{Z[a1,a2,a3,a4,a5,a6,a7,a8,a9,a10,a11,a12]}( J_199 ) gap> e := EmbedInSmallerAmbientSpace( m ); V_{Z[a9]}( I ) \ V_{Z[a9]}( J_1 ) \ V_{Z[a9]}( J_2 ) gap> Display( e ); V( <2*a9^2-2*a9+1> ) \ V( <5,a9+1> ) \ V( <5,a9-2> ) gap> a := DistinguishedLocallyClosedPart( e ); V_{Z[a9]}( I ) \ V_{Z[a9]}( J_1 ) \ V_{Z[a9]}( J_2 ) gap> Display( a ); V( <2*a9^2-2*a9+1> ) \ V( <3*a9-1> ) \ V( <a9+1> ) gap> piter := PseudoIteratorOfClosedPoints( e ); <iterator of closed points of V_{Z[a9]}( I ) \ V_{Z[a9]}( J_1 ) \ V_{Z[a9]}( J_2 )> gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(3)[a9]/( a9^2-a9-1 ): 1,0,1,1, 1, 0,1,1, 1, 0, 1, 1, 0, 1, 0,1,1,-a9-1,-a9,0,0,0, 0, 1, a9-1,1, 1, 1, 0,0,0,0, 0, 1,1,a9+1,-a9+1,a9,1, a9,-a9-1,1 modulo [ a9^2-a9-1 ] gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(7)[a9]/( a9^2-a9-3 ): 1,0,1,1, 1, 0,1,1, 1, 0, 1, 1, 0, 1, 0,1,1,2*a9-1,2*a9,0,0,0, 0, 1, -2*a9+2,1, 1, 1, 0,0,0,0, 0, 1,1,-2*a9+1,-a9+1,a9,1, a9,2*a9-1,1 modulo [ a9^2-a9-3 ] gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(11)[a9]/( a9^2-a9-5 ): 1,0,1,1, 1, 0,1,1, 1, 0, 1, 1, 0, 1, 0,1,1,2*a9-1,2*a9,0,0,0, 0, 1, -2*a9+2,1, 1, 1, 0,0,0,0, 0, 1,1,-2*a9+1,-a9+1,a9,1, a9,2*a9-1,1 modulo [ a9^2-a9-5 ] gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(13): 1,0,1,1, 1, 0,1,1,1,0, 1,1, 0, 1, 0,1,1,-5,-4,0,0,0,0,1, 6,1, 1, 1, 0,0,0,0, 0, 1,1,5,3,-2,1,-2,-5,1 gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(17): 1,0,1,1,1,0,1,1, 1,0, 1, 1, 0,1, 0,1,1,4,5,0,0,0, 0,1, -3,1, 1,1, 0,0,0,0,0,1,1,-4,7,-6,1, -6,4,1 gap> p := NextIterator( piter ); <A vector matroid> gap> Display( p ); The vector matroid of this matrix over GF(19)[a9]/( a9^2-a9-9 ): 1,0,1,1, 1, 0,1,1, 1, 0, 1, 1, 0, 1, 0,1,1,2*a9-1,2*a9,0,0,0, 0, 1, -2*a9+2,1, 1, 1, 0,0,0,0, 0, 1,1,-2*a9+1,-a9+1,a9,1, a9,2*a9-1,1 modulo [ a9^2-a9-9 ] gap> charset := ConstructibleProjection( e ); ( V_{Z}( I1 ) \ V_{Z}( J1_1 ) \ V_{Z}( J1_2 ) ) gap> Display( charset ); ( V( <> ) \ V( <2> ) \ V( <5> ) )
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