Singer difference sets and difference
system of sets
Akihiro Munemasa
Graduate School of Information Sciences
Tohoku University
(joint work with Vladimir D. Tonchev)
November 18, 2004
Singer difference sets and difference system of sets – p.1/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
Singer difference sets and difference system of sets – p.2/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
“point” = projective point
1-dim. vector subspace
Singer difference sets and difference system of sets – p.2/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
“point” = projective point
1-dim. vector subspace
“line” = projective line
2-dim. vector subspace
Singer difference sets and difference system of sets – p.2/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
“point” = projective point
1-dim. vector subspace
“line” = projective line
2-dim. vector subspace
“spread” = a set of lines which partition the points of
P G(n, q)
Singer difference sets and difference system of sets – p.2/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
“point” = projective point
1-dim. vector subspace
“line” = projective line
2-dim. vector subspace
“spread” = a set of lines which partition the points of
P G(n, q)
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of lines
Singer difference sets and difference system of sets – p.2/13
Projective Geometry
P G(n, q) = n-dim. projective space over GF (q)
(GF (q)n+1 − {0})/ ∼
“point” = projective point
1-dim. vector subspace
“line” = projective line
2-dim. vector subspace
“spread” = a set of lines which partition the points of
P G(n, q)
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of lines
∃ packing in P G(n, q) =⇒ n: odd ( ⇐= : open)
Singer difference sets and difference system of sets – p.2/13
P G(n, q), n: even
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of points
∃ packing in P G(n, q) =⇒ n: odd
Singer difference sets and difference system of sets – p.3/13
P G(n, q), n: even
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of points
∃ packing in P G(n, q) =⇒ n: odd
Question 1. Does there exist a partition of the set of
lines of P G(2n, q) into spreads of hyperplanes?
Singer difference sets and difference system of sets – p.3/13
P G(n, q), n: even
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of points
∃ packing in P G(n, q) =⇒ n: odd
Question 1. Does there exist a partition of the set of
lines of P G(2n, q) into spreads of hyperplanes?
When the answer to Question 1 is affirmative, we say
that P G(2n, q) is (2n − 1)-partitionable.
Singer difference sets and difference system of sets – p.3/13
P G(n, q), n: even
“packing” = “resolution” = “ parallelism” = a set of
spreads which partition the set of points
∃ packing in P G(n, q) =⇒ n: odd
Question 1. Does there exist a partition of the set of
lines of P G(2n, q) into spreads of hyperplanes?
When the answer to Question 1 is affirmative, we say
that P G(2n, q) is (2n − 1)-partitionable.
(q n+1 − 1)(q n − 1) (q n+1 − 1) (q n − 1)
#(lines) =
=
· 2
2
(q − 1)(q − 1)
(q − 1)
(q − 1)
lines in a spread
= #(hyperplanes) × #
of a hyperplane
Singer difference sets and difference system of sets – p.3/13
Fuji-hara,
(1986)
Jimbo
and
Vanstone
Question 2. Does there exist a spread SH for each
hyperplane H of P G(2n, q), such that
lines of P G(2n, q) =
SH (disjoint),
H
where H runs through all hyperplanes of P G(2n, q)?
Singer difference sets and difference system of sets – p.4/13
Fuji-hara,
(1986)
Jimbo
and
Vanstone
Question 2. Does there exist a spread SH for each
hyperplane H of P G(2n, q), such that
lines of P G(2n, q) =
SH (disjoint),
H
where H runs through all hyperplanes of P G(2n, q)?
Yes for (2n, q) = (4, 2), (4, 3), (6, q), etc.
Singer difference sets and difference system of sets – p.4/13
Fuji-hara,
(1986)
Jimbo
and
Vanstone
Question 2. Does there exist a spread SH for each
hyperplane H of P G(2n, q), such that
lines of P G(2n, q) =
SH (disjoint),
H
where H runs through all hyperplanes of P G(2n, q)?
Yes for (2n, q) = (4, 2), (4, 3), (6, q), etc.
The answer was unknown for (4, 4), (4, 5), (4, 7), etc.
Singer difference sets and difference system of sets – p.4/13
Singer Cycle
σ = Singer cycle of P G(2n, q)
q 2n+1 − 1
= cyclic automorphism of order
q−1
Singer difference sets and difference system of sets – p.5/13
Singer Cycle
σ = Singer cycle of P G(2n, q)
q 2n+1 − 1
= cyclic automorphism of order
q−1
only one orbit on points
σ has
only one orbit on hyperplanes
Singer difference sets and difference system of sets – p.5/13
Singer Cycle
σ = Singer cycle of P G(2n, q)
q 2n+1 − 1
= cyclic automorphism of order
q−1
only one orbit on points
σ has
only one orbit on hyperplanes
In P G(2n, q),
H = L1 ∪ L2 ∪ · · · ∪ Ls : spread of H
H σ = Lσ1 ∪ Lσ2 ∪ · · · ∪ Lσs : spread of H σ
..
.
Singer difference sets and difference system of sets – p.5/13
Orbits of Singer Cycle
In P G(2n, q),
H = L1 ∪ L2 ∪ · · · ∪ Ls : spread of H
H σ = Lσ1 ∪ Lσ2 ∪ · · · ∪ Lσs : spread of H σ
..
.
if distinct σ-orbits =⇒ (2n − 1)-partitionable
Singer difference sets and difference system of sets – p.6/13
Orbits of Singer Cycle
In P G(2n, q),
H = L1 ∪ L2 ∪ · · · ∪ Ls : spread of H
H σ = Lσ1 ∪ Lσ2 ∪ · · · ∪ Lσs : spread of H σ
..
.
if distinct σ-orbits =⇒ (2n − 1)-partitionable
Question 3. Does there exist a spread S of a hyperplane
H in P G(2n, q) such that the members of S belong to
distinct σ-orbits?
Singer difference sets and difference system of sets – p.6/13
Orbits of Singer Cycle
In P G(2n, q),
H = L1 ∪ L2 ∪ · · · ∪ Ls : spread of H
H σ = Lσ1 ∪ Lσ2 ∪ · · · ∪ Lσs : spread of H σ
..
.
if distinct σ-orbits =⇒ (2n − 1)-partitionable
Question 3. Does there exist a spread S of a hyperplane
H in P G(2n, q) such that the members of S belong to
distinct σ-orbits?
Such a spread produces a difference system of sets.
Singer difference sets and difference system of sets – p.6/13
Difference System of Sets
Suppose that there is a spread S of a hyperplane H of
P G(2n, q) such that the members of S belong to
different σ-orbits.
Singer difference sets and difference system of sets – p.7/13
Difference System of Sets
Suppose that there is a spread S of a hyperplane H of
P G(2n, q) such that the members of S belong to
different σ-orbits.
Then S becomes a difference system of sets, defined as
follows.
Singer difference sets and difference system of sets – p.7/13
Difference System of Sets
Suppose that there is a spread S of a hyperplane H of
P G(2n, q) such that the members of S belong to
different σ-orbits.
Then S becomes a difference system of sets, defined as
follows.
Definition. Let G be a finite group of order v, let λ, m be
positive integers.
Singer difference sets and difference system of sets – p.7/13
Difference System of Sets
Suppose that there is a spread S of a hyperplane H of
P G(2n, q) such that the members of S belong to
different σ-orbits.
Then S becomes a difference system of sets, defined as
follows.
Definition. Let G be a finite group of order v, let λ, m be
positive integers. A family of m-subsets
{B1 , B2 , . . . , Bk } of G is called a (v, k, λ; m) difference
system of sets if the multiset
{gh−1 | g ∈ Bi , h ∈ Bj , 1 ≤ i, j ≤ k, i = j}
coincides with λ(G − {1}).
Singer difference sets and difference system of sets – p.7/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
Singer difference sets and difference system of sets – p.8/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
{gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, i = j}
Singer difference sets and difference system of sets – p.8/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
{gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, i = j}
={gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, g = h}
−{gh−1 | g ∈ Li , h ∈ Li , 1 ≤ i ≤ k, g = h}
Singer difference sets and difference system of sets – p.8/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
{gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, i = j}
={gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, g = h}
−{gh−1 | g ∈ Li , h ∈ Li , 1 ≤ i ≤ k, g = h}
={gh−1 | g ∈ H, h ∈ H, g = h} difference set
k
− i=1 {gh−1 | g ∈ Li , h ∈ Li , g = h} difference
family
Singer difference sets and difference system of sets – p.8/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
{gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, i = j}
={gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, g = h}
−{gh−1 | g ∈ Li , h ∈ Li , 1 ≤ i ≤ k, g = h}
={gh−1 | g ∈ H, h ∈ H, g = h} difference set
k
− i=1 {gh−1 | g ∈ Li , h ∈ Li , g = h} difference
family
q n−1 −1
= q−1 (G − {1}) − (G − {1}).
Singer difference sets and difference system of sets – p.8/13
Partitionability and DSS
Indeed, identify σ with P G(2n, q). Then
{gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, i = j}
={gh−1 | g ∈ Li , h ∈ Lj , 1 ≤ i, j ≤ k, g = h}
−{gh−1 | g ∈ Li , h ∈ Li , 1 ≤ i ≤ k, g = h}
={gh−1 | g ∈ H, h ∈ H, g = h} difference set
k
− i=1 {gh−1 | g ∈ Li , h ∈ Li , g = h} difference
family
q n−1 −1
= q−1 (G − {1}) − (G − {1}).
Thus
q n+1 − 1 q n − 1 q n−1 − q
(
,
,
; q + 1) d.s.s.
q−1 q−1 q−1
Singer difference sets and difference system of sets – p.8/13
P G(4, 4)
Let H be a hyperplane in P G(4, 4). Define a graph Γ as
follows.
vertices = lines of H
Singer difference sets and difference system of sets – p.9/13
P G(4, 4)
Let H be a hyperplane in P G(4, 4). Define a graph Γ as
follows.
vertices = lines of H
edges = pairs {L, L } of skew lines such that
/ Lσ .
L ∈
Singer difference sets and difference system of sets – p.9/13
P G(4, 4)
Let H be a hyperplane in P G(4, 4). Define a graph Γ as
follows.
vertices = lines of H
edges = pairs {L, L } of skew lines such that
/ Lσ .
L ∈
Every clique of size q 2 + 1 = 17 in Γ gives a spread such
that its members belong to distinct σ-orbits.
Singer difference sets and difference system of sets – p.9/13
P G(4, 4)
Let H be a hyperplane in P G(4, 4). Define a graph Γ as
follows.
vertices = lines of H
edges = pairs {L, L } of skew lines such that
/ Lσ .
L ∈
Every clique of size q 2 + 1 = 17 in Γ gives a spread such
that its members belong to distinct σ-orbits.
Γ has
357 vertices, 42, 976 edges,
and using MAGMA, we see that Γ has no clique of size
17.
Singer difference sets and difference system of sets – p.9/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work.
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
• GF (q 5 ) ↔ GF (q)5 → P G(4, q)
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
• GF (q 5 ) ↔ GF (q)5 → P G(4, q)
• f = AutGF (q 5 ).
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
• GF (q 5 ) ↔ GF (q)5 → P G(4, q)
• f = AutGF (q 5 ).
Regard f as an automorphism of P G(4, q).
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
• GF (q 5 ) ↔ GF (q)5 → P G(4, q)
• f = AutGF (q 5 ).
Regard f as an automorphism of P G(4, q).
• f fixes a unique hyperplane H, but none of the lines of
H.
Singer difference sets and difference system of sets – p.10/13
P G(4, q) with q ≡ 2 or 3 (mod 5)
When q > 4, the exhaustive search like the case of
P G(4, 4) does not work. So we try to perform a more
restrictive search, by assuming more symmetry
(Frobenius automorphism).
• GF (q 5 ) ↔ GF (q)5 → P G(4, q)
• f = AutGF (q 5 ).
Regard f as an automorphism of P G(4, q).
• f fixes a unique hyperplane H, but none of the lines of
H.
Look for an f -invariant spread
S=
f
f2
f3
f4
f4
{L1 , L1 , L1 , L1 , L1 , . . . L q2 +1 }
5
of H, such that its members belong to distinct σ-orbits.
Singer difference sets and difference system of sets – p.10/13
P G(4, 8)
In graph theoretic terms again, define a graph Γ as
follows.
vertices = {Lf | L : line of H}: sets of skew lines
Singer difference sets and difference system of sets – p.11/13
P G(4, 8)
In graph theoretic terms again, define a graph Γ as
follows.
vertices = {Lf | L : line of H}: sets of skew lines
f edges = pairs {L , M
fi
/M
and L ∈
f j σ
f fi
} such that L ∩ M
fj
=∅
.
Singer difference sets and difference system of sets – p.11/13
P G(4, 8)
In graph theoretic terms again, define a graph Γ as
follows.
vertices = {Lf | L : line of H}: sets of skew lines
f edges = pairs {L , M
fi
/M
and L ∈
f j σ
f fi
} such that L ∩ M
fj
=∅
.
Every clique of size (q 2 + 1)/5 = 13 in Γ gives an f invariant spread such that its members belong to distinct
σ-orbits.
Singer difference sets and difference system of sets – p.11/13
P G(4, 8)
Γ has
715 vertices, 107, 694 edges,
Singer difference sets and difference system of sets – p.12/13
P G(4, 8)
Γ has
715 vertices, 107, 694 edges,
and using MAGMA, we see that Γ has a clique of size
13.
Singer difference sets and difference system of sets – p.12/13
P G(4, 8)
Γ has
715 vertices, 107, 694 edges,
and using MAGMA, we see that Γ has a clique of size
13.
Theorem. P G(4, 8) is 3-partitionable.
Singer difference sets and difference system of sets – p.12/13
P G(4, 8)
Γ has
715 vertices, 107, 694 edges,
and using MAGMA, we see that Γ has a clique of size
13.
Theorem. P G(4, 8) is 3-partitionable.
Somewhat more complicated analysis shows that
P G(4, q) is 3-partitionable for q = 5, 9.
Singer difference sets and difference system of sets – p.12/13
P G(4, 8)
Γ has
715 vertices, 107, 694 edges,
and using MAGMA, we see that Γ has a clique of size
13.
Theorem. P G(4, 8) is 3-partitionable.
Somewhat more complicated analysis shows that
P G(4, q) is 3-partitionable for q = 5, 9.
They give
q5 − 1 2
(v, k, λ) = (
, q + 1, q 2 + q; q + 1)
q−1
difference system of sets for q = 5, 8, 9.
Singer difference sets and difference system of sets – p.12/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
As before let σ denote a Singer cycle in P G(6, q).
Singer difference sets and difference system of sets – p.13/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
Question 4. Does there exist a spread Π of planes of a
hyperplane H in P G(6, q) such that
Singer difference sets and difference system of sets – p.13/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
Question 4. Does there exist a spread Π of planes of a
hyperplane H in P G(6, q) such that
• the members of Π belong to distinct σ-orbits,
Singer difference sets and difference system of sets – p.13/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
Question 4. Does there exist a spread Π of planes of a
hyperplane H in P G(6, q) such that
• the members of Π belong to distinct σ-orbits,
• Π forms a difference family.
Singer difference sets and difference system of sets – p.13/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
Question 4. Does there exist a spread Π of planes of a
hyperplane H in P G(6, q) such that
• the members of Π belong to distinct σ-orbits,
• Π forms a difference family.
If Π = {P1 , P2 , . . . , Pq3 +q } is such a spread of planes,
then Π forms a
q5 − q2 q3 − 1
q7 − 1 3
(
, q + 1,
;
)
q−1
q−1 q−1
difference system of sets.
Singer difference sets and difference system of sets – p.13/13
Spreads of Planes in P G(5, q) ⊂
P G(6, q)
Question 4. Does there exist a spread Π of planes of a
hyperplane H in P G(6, q) such that
• the members of Π belong to distinct σ-orbits,
• Π forms a difference family.
If Π = {P1 , P2 , . . . , Pq3 +q } is such a spread of planes,
then Π forms a
q5 − q2 q3 − 1
q7 − 1 3
(
, q + 1,
;
)
q−1
q−1 q−1
difference system of sets.
A difference family whose members belong to distinct
σ-orbits was constructed for q = 2 by Miyakawa–
Munemasa–Yoshiara (1995).
Singer difference sets and difference system of sets – p.13/13