Degree 8 Maximal Arcs in PG(2,2), h Odd

In a recent paper R. Mathon gave a new construction method for maximal arcs in finite Desarguesian projective planes that generalised a construction of Denniston. He also gave several instances of the method to construct new maximal arcs. In this paper, the structure of the maximal arcs is examined to give geometric and algebraic methods for proving when the maximal arcs are not of Denniston type. New degree 8 maximal arcs are also constructed in PG(2,2^h), h5, h odd. This, combined with previous results, shows that every Desarguesian projective plane of (even) order greater that 8 contains a degree 8 maximal arc that is not of Denniston type.     

An infinite family of symmetric designs

In this paper, using the construction method of [3], we show that if q>2 is a prime power such that there exists an affine plane of order q?1, then there exists a strongly divisible 2?(q?1)(q^h?1), q^h?1(q?1), q^h?1) design for every h?2. We show that these quasi-residual designs are embeddable, and hence establish the existence of an infinite family of symmetric 2?(q^h+1?q+1,q^h, q^h?1) designs. This construction may be regarded as a generalisation of the construction of [1, Chapter 4, Section 1] and [4].     

Bicovering arcs and small complete caps from elliptic curves

Bicovering arcs in Galois affine planes of odd order are a powerful tool for the construction of complete caps in spaces of arbitrarily higher dimensions. The aim of this paper is to investigate whether the arcs contained in elliptic cubic curves are bicovering. As a result, bicovering k-arcs in AG(2,q) of size k≤q/3 are obtained, provided that q?1 has a prime divisor m with 7<m<(1/8)q ^1/4. Such arcs produce complete caps of size kq ^(N?2)/2 in affine spaces of dimension N≡0(mod4). When q=p ^h with p prime and h≤8, these caps are the smallest known complete caps in AG(N,q), N≡0(mod4).     

Distance Regular Covers of Complete Graphs from Latin Squares

We describe a new construction of distance regular covers of a complete graph K_q^2t with fibres of size q^2t-1, q a power of 2. When q=2, the construction coincides with the one found in [D. de Caen, R. Mathon, G.E. Moorhouse. J. Algeb. Combinatorics, Vol. 4 (1995) 317] and studied in [T. Bending, D. Fon-Der-Flaass, Elect. J. Combinatorics, Vol. 5 (1998) R34]. The construction uses, as one ingredient, an arbitrary symmetric Latin square of order q; so, for large q, it can produce many different covers.     

Class number one quadratic fields and solvability of some Pellian equations

Consider these two types of positive square-free integers d≠ 1 for which the class number h of the quadratic field Q(√d) is odd: (1) d is prime∈ 1(mod 8), or d=2q where q is prime ≡ 3 (mod 4), or d=qr where q and r are primes such that q≡ 3 (mod 8) and r≡ 7 (mod 8); (2) d is prime ≡ 1 (mod 8), or d=qr where q and r are primes such that q≡r≡ 3 or 7 (mod 8). For d of type (2) (resp. (1)), let Π be the set of all primes (resp. odd primes) p∈N satisfying (d/p) = 1. Also, let δ :=0 (resp. δ :=1) if d≡ 2,3 (mod 4) (resp. d≡ 1 (mod 4)). Then the following are equivalent: (a) h=1; (b) For every p∈П at least one of the two Pellian equations Z ²-dY ² = ±4^δ p is solvable in integers. (c) For every p∈П the Pellian equation W ²-dV ² = 4^δ p ² has a solution (w,v) in integers such that gcd (w,v) divides 2^δ.     

The existence of maximal (q 2, 2)-arcs in projective Hjelmslev planes over chain rings of length 2 and odd prime characteristic

We prove that (q ², 2)-arcs exist in the projective Hjelmslev plane PHG(2, R) over a chain ring R of length 2, order |R| = q ² and prime characteristic. For odd prime characteristic, our construction solves the maximal arc problem. For characteristic 2, an extension of the above construction yields the lower bound q ² + 2 on the maximum size of a 2-arc in PHG(2, R). Translating the arcs into codes, we get linear [q ³, 6, q ³ ?q ² ?q] codes over ${\mathbb {F}_q}$ for every prime power q > 1 and linear [q ³ + q, 6,q ³ ?q ² ?1] codes over ${\mathbb {F}_q}$ for the special case q = 2 ^r . Furthermore, we construct 2-arcs of size (q + 1)²/4 in the planes PHG(2, R) over Galois rings R of length 2 and odd characteristic p ².     

Cyclic Arcs in PG(2, q)

B.C. Kestenband [9], J.C. Fisher, J.W.P. Hirschfeld, and J.A. Thas [3], E. Boros, and T. Szönyi [1] constructed complete (q ² ? q + l)-arcs in PG(2, q ²), q ≥ 3. One of the interesting properties of these arcs is the fact that they are fixed by a cyclic protective group of order q ² ? q + 1. We investigate the following problem: What are the complete k-arcs in PG(2, q) which are fixed by a cyclic projective group of order k? This article shows that there are essentially three types of those arcs, one of which is the conic in PG(2, q), q odd. For the other two types, concrete examples are given which shows that these types also occur.     

Transitive Arcs in Planes of Even Order

When one considers the hyperovals inPG(2,q),qeven,q>2, then the hyperoval inPG(2, 4) and the Lunelli-Sce hyperoval inPG(2, 16) are the only hyperovals stabilized by a transitive projective group [10]. In both cases, this group is an irreducible group fixing no triangle in the plane of the hyperoval, nor in a cubic extension of that plane. Using Hartley's classification of subgroups ofPGL₃(q),qeven [6], allk-arcs inPG(2,q) fixed by a transitive irreducible group, fixing no triangle inPG(2,q) or inPG(2,q³), are determined. This leads to new 18-, 36- and 72-arcs inPG(2,q),q=2^2h. The projective equivalences among the arcs are investigated and each section ends with a detailed study of the collineation groups of these arcs.     

Note on a conjecture of sierksma

LetS(q, d) be the maximal numberv such that, for every general position linear maph: Δ^(q?1)(d+1) →R ^d, there exist at leastv different collections {Δ ^t1, ..., Δ ^t _q} of disjoint faces of Δ(q?1)(d+1) with the property thatf(Δ ^t1) ∩ ... ∩f(Δ ^t _q) ≠ Ø. Sierksma's conjecture is thatS(q, d)=((q?1)!) ^d . The following lower bound (Theorem 1) is proved assuming thatq is a prime number: $$S(q,d) \geqslant \frac{1}{{(q - 1)!}}\left( {\frac{q}{2}} \right)^{{{((q - 1)(d + 1))} \mathord{\left/ {\vphantom {{((q - 1)(d + 1))} 2}} \right. \kern-\nulldelimiterspace} 2}} .$$ Using the same technique we obtain (Theorem 2) a lower bound for the number of different splittings of a “generic” necklace.     

Some inherited maximal arcs in derived dual translation planes

In 1974 J. A. Thas constructed a class of maximal arcs in certain translation planes of orderq ². In this paper a new class of maximal arcs is constructed in certain derived dual translation planes that are inherited from the duals of the Thas maximal arcs. It is noted that some (but not all) of the maximal arcs are isomorphic to a class constructed by the author.The author gratefully acknowledges the support of an Australian Postgraduate Research Award.     

Some New Maximal Sets of Mutually Orthogonal Latin Squares

Two ways of constructing maximal sets of mutually orthogonal Latin squares are presented. The first construction uses maximal partial spreads in PG(3, 4) \ PG(3, 2) with r lines, where r ∈ {6, 7}, to construct transversal-free translation nets of order 16 and degree r + 3 and hence maximal sets of r + 1 mutually orthogonal Latin squares of order 16. Thus sets of t MAXMOLS(16) are obtained for two previously open cases, namely for t = 7 and t = 8. The second one uses the (non)existence of spreads and ovoids of hyperbolic quadrics Q ⁺ (2m + 1, q), and yields infinite classes of q ^{2n ? 1} ? 1 MAXMOLS(q ²ⁿ ), for n ≥ 2 and q a power of two, and for n = 2 and q a power of three.     

Matrix Maps of Quasi-Convex Null Sequences

In [7] Stieglitz and Tietz identify the space q ^α of all quasi-convex convergent sequences as a BK-space. They characterize all infinite matrices which map q ^α into an arbitrary FK-space. In [6] they do so for matrices which map a particular class of sequence spaces into q ^α . In [10] Zygmund introduces q ² in connexion with convergence factors of Fourier series. Dawson considers in [3] and [4] matrix maps of the space q ₀ ^α of all quasi-convex null sequences. In Section 2 we characterize all matrices which map q ₀ ^α into an arbitrary FK-space. Prior to that, a particular matrix map on q ₀ ^α gives us the BK-topology on q ₀ ^α . As an application we characterize in Section 3 the matrices which map q ₀ ^α into the FK-spaces considered by Stieglitz and Tietz in [8]. Based on [6], we determine the matrices which map these spaces into q ₀ ^α . Using methods similar to those in [7] our results in Section 2 depend on Theorems 2.1 and 4.1 in [5] due to Jakimovski and Livne. Theorem 2.1 gives for suitable pairs of sequence spaces necessary and sufficient conditions for an infinite matrix to map one space into the second one. In Theorem 4.1 a special sequence which is useful in applications of quasi-convexity is constructed. We close our paper with two remarks concerning three results in [8].     

On the Embedding of (k,p)-Arcs is Maximal Arcs

Our main result is that a (k,p)-arc in PG (2,q),q = p ^h , with k qp - q + p - can be extended to a maximal arc. Combining this result with the recent Ball, Blokhuis, Mazzocca theorem about the non-existence of maximal arcs for p > 2, it gives an upper bound for the size of a (k,p)-arc. The method can be regarded as a generalization of B. Segre's method for proving similar embeddability theorems for k-arcs (that is when n= 2). It is based on associating an algebraic envelope containing the short lines to the (k,p)-arc. However, the construction of the envelope is independent of Segre's method using the generalization of Menelaus' theorem.     

Construction of steiner quadruple systems having a prescribed number of blocks in common

Let q_υ=υ(υ–1)(υ–2)/24 and let I_υ={0, 1, 2, …, q_{υ–14}∪{qυ}–12, q_υ–8, q_υ}, for υ?8 Further, let J[υ] denote the set of all k such that there exists a pair of Steiner quadruple systems of order υ having exactly k blocks in common. We determine J[υ] for all υ=2ⁿ, n?2, with the possible exception of 7 cases for υ=16 and of 5 cases for each υ?32. In particular we show: J[υ]?I_υ for all υ≡2 or 4 (mod 6) and υ?8, J[4]={1}, J[8]=I₈={0, 2, 6, 14}, I₁₆?{103, 111, 115, 119, 121, 122, 123}?J[16], and I_υ? {q_υ–h:h=17, 18, 19, 21, 25}?J[υ] for all υ=2ⁿ, n?5.     

Fast construction of irreducible polynomials over finite fields

We present a randomized algorithm that on inputting a finite field K with q elements and a positive integer d outputs a degree d irreducible polynomial in K[x]. The running time is d ^1+?(d)×(log q)^5+?(q) elementary operations. The function ? in this expression is a real positive function belonging to the class o(1), especially, the complexity is quasi-linear in the degree d. Once given such an irreducible polynomial of degree d, we can compute random irreducible polynomials of degree d at the expense of d ^1+?(d) × (log q)^1+?(q) elementary operations only.     

On sizes of complete caps in projective spaces PG(n, q) and arcs in planes PG(2, q)

More than thirty new upper bounds on the smallest size t ₂(2, q) of a complete arc in the plane PG(2, q) are obtained for (169 ≤ q ≤ 839. New upper bounds on the smallest size t ₂(n, q) of the complete cap in the space PG(n, q) are given for n = 3 and 25 ≤ q ≤ 97, q odd; n = 4 and q = 7, 8, 11, 13, 17; n = 5 and q = 5, 7, 8, 9; n = 6 and q = 4, 8. The bounds are obtained by computer search for new small complete arcs and caps. New upper bounds on the largest size m ₂(n, q) of a complete cap in PG(n, q) are given for q = 4, n = 5, 6, and q = 3, n = 7, 8, 9. The new lower bound 534 ≤ m ₂(8, 3) is obtained by finding a complete 534-cap in PG(8, 3). Many new sizes of complete arcs and caps are obtained. The updated tables of upper bounds for t ₂(n, q), n ≥ 2, and of the spectrum of known sizes for complete caps are given. Interesting complete caps in PG(3, q) of large size are described. A proof of the construction of complete caps in PG(3, 2 ^h ) announced in previous papers is given; this is modified from a construction of Segre. In PG(2, q), for q = 17, δ = 4, and q = 19, 27, δ = 3, we give complete ${(\frac{1}{2}(q + 3) + \delta)}$ -arcs other than conics that share ${\frac{1}{2}(q + 3)}$ points with an irreducible conic. It is shown that they are unique up to collineation. In PG(2, q), ${{q \equiv 2}}$ (mod 3) odd, we propose new constructions of ${\frac{1}{2} (q + 7)}$ -arcs and show that they are complete for q ≤ 3701.     

On large maximal partial ovoids of the parabolic quadric Q(4, q)

We use the representation ${T_2(\mathcal{O})}$ for Q(4, q) to show that maximal partial ovoids of Q(4, q) of size q ² ? 1, q = p ^h , p an odd prime, h > 1, do not exist. Although this was known before, we give a slightly alternative proof, also resulting in more combinatorial information of the known examples for q an odd prime.     

A new characterization for some extensions of PSL(2, <Emphasis Type="Italic">q)</Emphasis> for some <Emphasis Type="Italic">q</Emphasis> by some character degrees

In [22] (Tong-Viet H P, Simple classical groups of Lie type are determined by their character degrees, J. Algebra, 357 (2012) 61–68), the following question arose: Which groups can be uniquely determined by the structure of their complex group algebras? The authors in [12] (Khosravi B et al., Some extensions of PSL(2,p²) are uniquely determined by their complex group algebras, Comm. Algebra, 43(8) (2015) 3330–3341) proved that each extension of PSL(2,p²) of order 2|PSL(2,p²)| is uniquely determined by its complex group algebra. In this paper we continue this work. Let p be an odd prime number and q = p or q = p³. Let M be a finite group such that |M| = h|PSL(2,q), where h is a divisor of |Out(PSL(2,q))|. Also suppose that M has an irreducible character of degree q and 2p does not divide the degree of any irreducible character of M. As the main result of this paper we prove that M has a unique nonabelian composition factor which is isomorphic to PSL(2,q). As a consequence of our result we prove that M is uniquely determined by its order and some information on its character degrees which implies that M is uniquely determined by the structure of its complex group algebra.