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PROPERTIES OF SOME ALGEBRAICALLYDEFINED DIGRAPHS
Aleksandr Kodess, Felix Lazebnik
Department of Mathematical SciencesUniversity of Delaware
Modern Trends of Algebraic Graph Theory
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is a digraph?
v1
v2
v3
v4
v5DefinitionA digraph is a pair D = (V ,A)of:
a set V , whose elementsare called vertices ornodesa set A of ordered pairs ofvertices, called arcs,directed edges, or arrows
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is an algebraic digraph D(q; f )?
LetFq be a finite field with q elements;f : F2
q → Fq be a bivariate polynomial.
DefinitionAn algebraic digraph, denoted D(q; f ), is a digraph whose
vertex set is F2q
arc set consits of ordered pairs(
(x1, x2), (y1, y2))
with therelation
x2 + y2 = f (x1, y1),
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is an algebraic digraph D(q; f )?
LetFq be a finite field with q elements;f : F2
q → Fq be a bivariate polynomial.
DefinitionAn algebraic digraph, denoted D(q; f ), is a digraph whose
vertex set is F2q
arc set consits of ordered pairs(
(x1, x2), (y1, y2))
with therelation
x2 + y2 = f (x1, y1),
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is an algebraic digraph D(q; f )?
LetFq be a finite field with q elements;f : F2
q → Fq be a bivariate polynomial.
DefinitionAn algebraic digraph, denoted D(q; f ), is a digraph whose
vertex set is F2q
arc set consits of ordered pairs(
(x1, x2), (y1, y2))
with therelation
x2 + y2 = f (x1, y1),
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is an algebraic digraph D(q; f )?
LetFq be a finite field with q elements;f : F2
q → Fq be a bivariate polynomial.
DefinitionAn algebraic digraph, denoted D(q; f ), is a digraph whose
vertex set is F2q
arc set consits of ordered pairs(
(x1, x2), (y1, y2))
with therelation
x2 + y2 = f (x1, y1),
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is an algebraic digraph D(q; f )?
LetFq be a finite field with q elements;f : F2
q → Fq be a bivariate polynomial.
DefinitionAn algebraic digraph, denoted D(q; f ), is a digraph whose
vertex set is F2q
arc set consits of ordered pairs(
(x1, x2), (y1, y2))
with therelation
x2 + y2 = f (x1, y1),
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Example of D(q; f )
Example of D(q; f )
V (D) = F2q
f : F2q → Fq
There is an arc from vertex (x1, x2) to vertex (y1, y2) if andonly if
x2 + y2 = x2 + xy + y2 + 1 = f1(x1, y1)x2 + y2 = x2 + xy + y2 = f2(x1, y1)x2 + y2 = xy = f3(x1, y1)
Easy to argue that if q is odd, then
D(q; f1) ∼= D(q; f2) ∼= D(q; f3).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Example of D(q; f )
Example of D(q; f )
V (D) = F2q
f : F2q → Fq
There is an arc from vertex (x1, x2) to vertex (y1, y2) if andonly if
x2 + y2 = x2 + xy + y2 + 1 = f1(x1, y1)x2 + y2 = x2 + xy + y2 = f2(x1, y1)x2 + y2 = xy = f3(x1, y1)
Easy to argue that if q is odd, then
D(q; f1) ∼= D(q; f2) ∼= D(q; f3).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Example of D(q; f )
Example of D(q; f )
V (D) = F2q
f : F2q → Fq
There is an arc from vertex (x1, x2) to vertex (y1, y2) if andonly if
x2 + y2 = x2 + xy + y2 + 1 = f1(x1, y1)x2 + y2 = x2 + xy + y2 = f2(x1, y1)x2 + y2 = xy = f3(x1, y1)
Easy to argue that if q is odd, then
D(q; f1) ∼= D(q; f2) ∼= D(q; f3).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Example of D(q; f )
Example of D(q; f )
V (D) = F2q
f : F2q → Fq
There is an arc from vertex (x1, x2) to vertex (y1, y2) if andonly if
x2 + y2 = x2 + xy + y2 + 1 = f1(x1, y1)x2 + y2 = x2 + xy + y2 = f2(x1, y1)x2 + y2 = xy = f3(x1, y1)
Easy to argue that if q is odd, then
D(q; f1) ∼= D(q; f2) ∼= D(q; f3).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Simple Observation
Simple isomorphisms
Let q be an odd prime power, and f ∈ Fq[x , y ]. Letf1(x , y) = f (x , y)− f (0,0), andf ∗(x , y) = f1(x , y)− f (x ,0)− f (0, y). The following statementshold:
D(q; f ) ∼= D(q; f1).If, in addition, f is a symmetric polynomial, then
D(q; f ) ∼= D(q; f1) ∼= D(q; f ∗).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Simple Observation
Simple isomorphisms
Let q be an odd prime power, and f ∈ Fq[x , y ]. Letf1(x , y) = f (x , y)− f (0,0), andf ∗(x , y) = f1(x , y)− f (x ,0)− f (0, y). The following statementshold:
D(q; f ) ∼= D(q; f1).If, in addition, f is a symmetric polynomial, then
D(q; f ) ∼= D(q; f1) ∼= D(q; f ∗).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Simple Observation
Simple isomorphisms
Let q be an odd prime power, and f ∈ Fq[x , y ]. Letf1(x , y) = f (x , y)− f (0,0), andf ∗(x , y) = f1(x , y)− f (x ,0)− f (0, y). The following statementshold:
D(q; f ) ∼= D(q; f1).If, in addition, f is a symmetric polynomial, then
D(q; f ) ∼= D(q; f1) ∼= D(q; f ∗).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
What is a monomial algebraic digraph?
DefinitionA monomial algebraic digraph, denoted D(q; m,n), is analgebraic digraph in which
vertex set V is F2q
there is an arc from (x1, x2) to (y1, y2) if and only if
x2 + y2 = xm1 yn
1 .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
D(3; 1,2)
H0,1L
H0,2L
H1,2L
H2,2L
H1,1L
H2,1L
H2,0L
H1,0L
H0,0L
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Motivation
Work of:Lazebnik, Woldar (2001)Lazebnik, Ustimenko (1993, 1995, 1996)Viglione (2001)Dmytrenko, Lazebnik, Viglione (2005)
Bipartite undirected graph BΓn
V (BΓn) = Pn ∪ Ln, both Pn and Ln are copies of Fnq
point (p) = (p1, . . . , pn) is adjacent to line [l] = (l1, . . . , ln) if
l2 + p2 = f2(p1, l1)l3 + p3 = f3(p1, l1,p2, l2)
...ln + pn = fn(p1, l1,p2, l2, . . . , pn−1, ln−1).
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Properties of BΓn
BΓn admits neighbor-complete coloring, i.e. every color isuniquely represented among the neighbors of each vertexcovering properties of BΓn. For instance, BΓn covers BΓkfor n > k .embedded spectra properties. For instance,spec(BΓk ) ⊆ spec(BΓn) for k < n.edge-decompostiion properties. We have BΓndecomposing Kqn,qn .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Properties of BΓn
BΓn admits neighbor-complete coloring, i.e. every color isuniquely represented among the neighbors of each vertexcovering properties of BΓn. For instance, BΓn covers BΓkfor n > k .embedded spectra properties. For instance,spec(BΓk ) ⊆ spec(BΓn) for k < n.edge-decompostiion properties. We have BΓndecomposing Kqn,qn .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Properties of BΓn
BΓn admits neighbor-complete coloring, i.e. every color isuniquely represented among the neighbors of each vertexcovering properties of BΓn. For instance, BΓn covers BΓkfor n > k .embedded spectra properties. For instance,spec(BΓk ) ⊆ spec(BΓn) for k < n.edge-decompostiion properties. We have BΓndecomposing Kqn,qn .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Properties of BΓn
BΓn admits neighbor-complete coloring, i.e. every color isuniquely represented among the neighbors of each vertexcovering properties of BΓn. For instance, BΓn covers BΓkfor n > k .embedded spectra properties. For instance,spec(BΓk ) ⊆ spec(BΓn) for k < n.edge-decompostiion properties. We have BΓndecomposing Kqn,qn .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
An application of BΓn with a certain specialization
LetCn denote the cycle of length n ≥ 3ex(v , {C3,C4, . . . ,C2k}) denote the greatest number ofedges in a graph or order v which contains no subgraphsisomorphic to any C3, . . . ,C2k .
Theorem (Lazebnik, Ustimenko, Woldar 1995)
ex(v , {C3,C4, . . . ,C2k}) ≥ ckv1+ 23k−3+ε ,
where ck is a positive function if k, and ε = 0 if k 6= 5 is odd,and ε = 1 if k is even.
This lower bounds comes from BΓn with a certain choice ofdefining functions fi , 2 ≤ i ≤ n.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
An application of BΓn with a certain specialization
LetCn denote the cycle of length n ≥ 3ex(v , {C3,C4, . . . ,C2k}) denote the greatest number ofedges in a graph or order v which contains no subgraphsisomorphic to any C3, . . . ,C2k .
Theorem (Lazebnik, Ustimenko, Woldar 1995)
ex(v , {C3,C4, . . . ,C2k}) ≥ ckv1+ 23k−3+ε ,
where ck is a positive function if k, and ε = 0 if k 6= 5 is odd,and ε = 1 if k is even.
This lower bounds comes from BΓn with a certain choice ofdefining functions fi , 2 ≤ i ≤ n.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some Upper Bounds on ex(v , {C3,C4, . . . ,C2k})
Erdos, Bondy–Simonovits, 1974
ex(v , {C3,C4, . . . ,C2k}) ≤ 20kv1+(1/k) , for v sufficiently large.
Verstraëte, 2000
ex(v , {C3,C4, . . . ,C2k}) ≤ 8(k − 1)v1+(1/k), for v sufficientlylarge.
Pikhurko, 2012
ex(v , {C3,C4, . . . ,C2k}) ≤ (k − 1)v1+(1/k) + O(v) .
Bukh, Jiang, 2014
ex(v , {C3,C4, . . . ,C2k}) ≤ 80√
k log k · v1+(1/k) + O(v) .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some Upper Bounds on ex(v , {C3,C4, . . . ,C2k})
Erdos, Bondy–Simonovits, 1974
ex(v , {C3,C4, . . . ,C2k}) ≤ 20kv1+(1/k) , for v sufficiently large.
Verstraëte, 2000
ex(v , {C3,C4, . . . ,C2k}) ≤ 8(k − 1)v1+(1/k), for v sufficientlylarge.
Pikhurko, 2012
ex(v , {C3,C4, . . . ,C2k}) ≤ (k − 1)v1+(1/k) + O(v) .
Bukh, Jiang, 2014
ex(v , {C3,C4, . . . ,C2k}) ≤ 80√
k log k · v1+(1/k) + O(v) .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some Upper Bounds on ex(v , {C3,C4, . . . ,C2k})
Erdos, Bondy–Simonovits, 1974
ex(v , {C3,C4, . . . ,C2k}) ≤ 20kv1+(1/k) , for v sufficiently large.
Verstraëte, 2000
ex(v , {C3,C4, . . . ,C2k}) ≤ 8(k − 1)v1+(1/k), for v sufficientlylarge.
Pikhurko, 2012
ex(v , {C3,C4, . . . ,C2k}) ≤ (k − 1)v1+(1/k) + O(v) .
Bukh, Jiang, 2014
ex(v , {C3,C4, . . . ,C2k}) ≤ 80√
k log k · v1+(1/k) + O(v) .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some Upper Bounds on ex(v , {C3,C4, . . . ,C2k})
Erdos, Bondy–Simonovits, 1974
ex(v , {C3,C4, . . . ,C2k}) ≤ 20kv1+(1/k) , for v sufficiently large.
Verstraëte, 2000
ex(v , {C3,C4, . . . ,C2k}) ≤ 8(k − 1)v1+(1/k), for v sufficientlylarge.
Pikhurko, 2012
ex(v , {C3,C4, . . . ,C2k}) ≤ (k − 1)v1+(1/k) + O(v) .
Bukh, Jiang, 2014
ex(v , {C3,C4, . . . ,C2k}) ≤ 80√
k log k · v1+(1/k) + O(v) .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Questions studied
Strong Connectivity of D(q; f )
Classification into Isomorphism Classes
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Questions studied
Strong Connectivity of D(q; f )
Classification into Isomorphism Classes
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Strong Connectivity
DefinitionA directed graph is calledstrongly connected if it containsa directed path from u to vand a directed path from v tou for every pair of distinctvertices u, v .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
D(3; 1,2) revisited
H0,1L
H0,2L
H1,2L
H2,2L
H1,1L
H2,1L
H2,0L
H1,0L
H0,0L
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
(Strong) Connectivity of Algebraic Digraphs
Definition (Linked alternating sums)
For any function f : F2q → Fq, any positive integer k ≥ 2 and any
k -tuple (x1, . . . , xk ) ∈ Fkq we call the sum
f (x1, x2)− f (x2, x3) + · · ·+ (−1)k−1f (xk−1, xk )
a linked alternating sum of f of length k − 1. Let LSk (f ) be theset of all possible linked alternating sums of f of length k − 1,and let
LS(f ) :=⋃k≥2
LSk (f ).
We call LS(f ) the set of linked alternating sums of f .
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some additional notation
Im(f ) — image of f : F2q → Fq
Im(f ) := {(f (x , y), : x , y ∈ Fq}.
Affine subspaces H0 and H1
For any vectors α1, . . . , αd ∈ Fq, let H0 and H1 be defined as
H0 = H0(α1, . . . , αd ) =
{d∑
i=1
siαi : all si in Fp,
d∑i=1
si = 0
},
H1 = H1(α1, . . . , αd ) =
{d∑
i=1
siαi : all si in Fp,
d∑i=1
si = 1
}.
Then H0 is a subspace of Fq, and H1 is an affine subspace ofFq.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Property of the set LS(f)
Trivially, LS(f ) ⊆ 〈Im(f )〉
EitherLS(f ) = 〈Im(f )〉, span of image of f in Fq, OR
LS(f ) = H0 ∪ H1, union of two affine planes in Fq
This leads to the following
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Property of the set LS(f)
Trivially, LS(f ) ⊆ 〈Im(f )〉
EitherLS(f ) = 〈Im(f )〉, span of image of f in Fq, OR
LS(f ) = H0 ∪ H1, union of two affine planes in Fq
This leads to the following
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Property of the set LS(f)
Trivially, LS(f ) ⊆ 〈Im(f )〉
EitherLS(f ) = 〈Im(f )〉, span of image of f in Fq, OR
LS(f ) = H0 ∪ H1, union of two affine planes in Fq
This leads to the following
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Strong Connectivity of D(p; f )
TheoremLet p be an odd prime, and let f : Fp × Fp → Fp. The followingstatements hold.
(i) If f is not a symmetric function, i.e. there exist x1, x2 ∈ Fpfor which f (x1, x2) 6= f (x2, x1), then D(p; f ) is strong.
(ii) If f is a symmetric function, let
f ∗(x , y) = f (x , y)− f (x ,0)− f (0, y)− f (0,0).
If f ∗ is a nonzero polynomial, then D(p; f ) is strong. If f ∗ isthe zero polynomial, then D(p; f ) has (p + 1)/2 strongcomponents. The component containing the vertex (0,0) isisomorphic to the complete looped digraph
−→K p. All other
strong components are isomorphic to the completebipartite digraph
−→K p,p with no loops.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
ProblemWhat are necessary and sufficient conditions for (m1,n1) and(m2,n2) in order for the graphs D(q; m1,n1) and D(q; m2,n2) tobe isomorphic?
Idea to Solve it — find a strong easily computable digraphinvariant
number of cyclesnumber of ’triangles’ with various orientations of arcsnumber of subdigraphs on 4 verticesnumber of ’looped’ paths
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
Dmytrenko, Lazebnik,Viglione, 2005 — undirected bipartiteversionThis worked for undirected bipartite monomial graphs. Theyfound the following simple graph invariants:
number of C4 for all sufficiently large q’snumber of Ks,t for all q’s.as multisets {m1,n1} = {m2,n2}, where x = gcd(q − 1, x)
Directed versionWe have examples of non-isomorphic graphs that have equalnumbers of 3,4,5,6-cycles and a large collection of othersubgraphs.
Example: Digraphs D(169; 1,5) and D(169; 1,101) have equalnumber of directed cycles Ck , for 3 ≤ k ≤ 13, but they are notisomorphic.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
Dmytrenko, Lazebnik,Viglione, 2005 — undirected bipartiteversionThis worked for undirected bipartite monomial graphs. Theyfound the following simple graph invariants:
number of C4 for all sufficiently large q’snumber of Ks,t for all q’s.as multisets {m1,n1} = {m2,n2}, where x = gcd(q − 1, x)
Directed versionWe have examples of non-isomorphic graphs that have equalnumbers of 3,4,5,6-cycles and a large collection of othersubgraphs.
Example: Digraphs D(169; 1,5) and D(169; 1,101) have equalnumber of directed cycles Ck , for 3 ≤ k ≤ 13, but they are notisomorphic.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Isomorphism Classification Problem
Dmytrenko, Lazebnik,Viglione, 2005 — undirected bipartiteversionThis worked for undirected bipartite monomial graphs. Theyfound the following simple graph invariants:
number of C4 for all sufficiently large q’snumber of Ks,t for all q’s.as multisets {m1,n1} = {m2,n2}, where x = gcd(q − 1, x)
Directed versionWe have examples of non-isomorphic graphs that have equalnumbers of 3,4,5,6-cycles and a large collection of othersubgraphs.
Example: Digraphs D(169; 1,5) and D(169; 1,101) have equalnumber of directed cycles Ck , for 3 ≤ k ≤ 13, but they are notisomorphic.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Open Conjecture
Conjecture
Let q be a prime power, 1 ≤ m,n,m′,n′ ≤ q − 1 integers. ThenD(q; m1,n1) ∼= D(q; m2,n2) if and only if there exists k coprimewith q − 1 such that
m2 ≡ km1 mod q − 1,
n2 ≡ kn1 mod q − 1.
Commentseasy to prove sufficiencycannot prove necessitychecked with sage for orders of up to 1000
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Open Conjecture
Conjecture
Let q be a prime power, 1 ≤ m,n,m′,n′ ≤ q − 1 integers. ThenD(q; m1,n1) ∼= D(q; m2,n2) if and only if there exists k coprimewith q − 1 such that
m2 ≡ km1 mod q − 1,
n2 ≡ kn1 mod q − 1.
Commentseasy to prove sufficiencycannot prove necessitychecked with sage for orders of up to 1000
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Open Conjecture
Conjecture
Let q be a prime power, 1 ≤ m,n,m′,n′ ≤ q − 1 integers. ThenD(q; m1,n1) ∼= D(q; m2,n2) if and only if there exists k coprimewith q − 1 such that
m2 ≡ km1 mod q − 1,
n2 ≡ kn1 mod q − 1.
Commentseasy to prove sufficiencycannot prove necessitychecked with sage for orders of up to 1000
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Open Conjecture
Conjecture
Let q be a prime power, 1 ≤ m,n,m′,n′ ≤ q − 1 integers. ThenD(q; m1,n1) ∼= D(q; m2,n2) if and only if there exists k coprimewith q − 1 such that
m2 ≡ km1 mod q − 1,
n2 ≡ kn1 mod q − 1.
Commentseasy to prove sufficiencycannot prove necessitychecked with sage for orders of up to 1000
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some comments on Conjecture
Using reduction to bipartites graphs
D(q; m1,n1) ∼= D(q; m2,n2) implies
{m1,n1} = {m2,n2}
If, say, m1 = m2 and n1 = n2, then
m2 ≡ k1m1 mod q − 1,
n2 ≡ k2n1 mod q − 1.
We cannot conclude k1 = k2. However, we have noexamples of isomorphic digraphs with k1 6= k2.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some comments on Conjecture
Using reduction to bipartites graphs
D(q; m1,n1) ∼= D(q; m2,n2) implies
{m1,n1} = {m2,n2}
If, say, m1 = m2 and n1 = n2, then
m2 ≡ k1m1 mod q − 1,
n2 ≡ k2n1 mod q − 1.
We cannot conclude k1 = k2. However, we have noexamples of isomorphic digraphs with k1 6= k2.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Some comments on Conjecture
Using reduction to bipartites graphs
D(q; m1,n1) ∼= D(q; m2,n2) implies
{m1,n1} = {m2,n2}
If, say, m1 = m2 and n1 = n2, then
m2 ≡ k1m1 mod q − 1,
n2 ≡ k2n1 mod q − 1.
We cannot conclude k1 = k2. However, we have noexamples of isomorphic digraphs with k1 6= k2.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs
Conjecture proved under restrictions for p > 2
TheoremLet q = p > 2 be prime and 1 ≤ m1,n1,m2,n2 ≤ p − 1 beintegers. Suppose that D1 = D(p; m1,n1) ∼= D2 = D(p; m2,n2).Assume moreover that there exists an isomorphismφ : V (D1)→ V (D2) of the form
φ : (x , y) 7→ (f (x , y),g(x , y)),
in which f depends on x only. If m1 6= n1 or m2 6= n2, then thereexists an integer k, coprime with p − 1, such that
m2 ≡ km1 mod p − 1,
n2 ≡ kn1 mod p − 1.
Aleksandr Kodess, Felix Lazebnik Algebraic Digraphs