[design] 2 Information from design parameters

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# 2 Information from design parameters

### Sections

1. Information from $t$-design parameters
2. Information from (mixed) orthogonal array parameters
3. Block intersection polynomials

## 2.1 Information from $t$-design parameters

For t a non-negative integer and v,k,lambda positive integers with tleklev, a t-design with parameters t,v,k,lambda, or a t-(v,k,lambda) design, is a binary block design with exactly v points, such that each block has size k and each t-subset of the points is contained in exactly lambda blocks.

- `TDesignLambdas(` t`,` v`,` k`,` lambda`)`

  A t-(v,k,lambda) design is also an s-(v,k,lambdas) design for 0leslet, where lambdas=lambdav-s chooset-s/k-s chooset-s.

  Given a non-negative integer t, and positive integers v, k, lambda, with tleklev, this function returns a length t+1 list whose (s+1)-st element is lambdas as defined above, if all the lambdas are integers. Otherwise, `fail` is returned.

      gap> TDesignLambdas(5,24,8,1);
      [ 759, 253, 77, 21, 5, 1 ]

- `TDesignLambdaMin(` t`,` v`,` k`)`

  Given a non-negative integer t, and positive integers v and k, with tleklev, this function returns the minimum positive lambda such that `TDesignLambdas(` t`,` v`,` k`,` lambda`)` does not return `fail`.

  See TDesignLambdas.

      gap> TDesignLambdaMin(5,24,8);
      1
      gap> TDesignLambdaMin(2,12,4);
      3

- `TDesignIntersectionTriangle(` t`,` v`,` k`,` lambda`)`

  Suppose D is a t-(v,k,lambda) design, let i and j be non-negative integers with i+jlet, and suppose X and Y are disjoint subsets of the points of D, such that X and Y have respective sizes i and j. The (i,j)-intersection number is the number of blocks of D that contain X and are disjoint from Y (this number depends only on t, v, k, lambda, i and j).

  Given a non-negative integer t, and positive integers v, k and lambda, with tleklev, this function returns the t-design intersection triangle, which is a two dimensional array whose (i+1,j+1)-entry is the (i,j)-intersection number for a t-(v,k,lambda) design (assuming such a design exists), such that i,jge0, i+jlet. This function returns `fail` if `TDesignLambdas(`t`,`v`,`k`,`lambda`)` does. When lambda=1, then more information can be obtained using SteinerSystemIntersectionTriangle.

      gap> TDesignLambdas(2,12,4,3);
      [ 33, 11, 3 ]
      gap> TDesignIntersectionTriangle(2,12,4,3);
      [ [ 33, 22, 14 ], [ 11, 8 ], [ 3 ] ]
      gap> TDesignLambdas(2,12,4,2);
      fail
      gap> TDesignIntersectionTriangle(2,12,4,2);
      fail

- `SteinerSystemIntersectionTriangle(` t`,` v`,` k`)`

  A Steiner system is a t-(v,k,1) design, and in this case it is possible to extend the notion of intersection triangle defined in TDesignIntersectionTriangle.

  Suppose D is a t-(v,k,1) design, with B a block of D, let i and j be non-negative integers with i+jlek, and suppose X and Y are disjoint subsets of B, such that X and Y have respective sizes i and j. The (i,j)-intersection number is the number of blocks of D that contain X and are disjoint from Y (this number depends only on t, v, k, i and j). Note that when i+jlet, this intersection number is the same as that defined in TDesignIntersectionTriangle for the general t-design case.

  Given a non-negative integer t, and positive integers v and k, with tleklev, this function returns the Steiner system intersection triangle, which is a two dimensional array whose (i+1,j+1)-entry is the (i,j)-intersection number for a t-(v,k,1) design (assuming such a design exists), such that i,jge0, i+jle k. This function returns `fail` if `TDesignLambdas(`t`,`v`,`k`,1)` does.

  See also TDesignIntersectionTriangle.

      gap> SteinerSystemIntersectionTriangle(5,24,8);
      [ [ 759, 506, 330, 210, 130, 78, 46, 30, 30 ],
        [ 253, 176, 120, 80, 52, 32, 16, 0 ], [ 77, 56, 40, 28, 20, 16, 16 ],
        [ 21, 16, 12, 8, 4, 0 ], [ 5, 4, 4, 4, 4 ], [ 1, 0, 0, 0 ], [ 1, 0, 0 ],
        [ 1, 0 ], [ 1 ] ]
      gap> TDesignIntersectionTriangle(5,24,8,1);
      [ [ 759, 506, 330, 210, 130, 78 ], [ 253, 176, 120, 80, 52 ],
        [ 77, 56, 40, 28 ], [ 21, 16, 12 ], [ 5, 4 ], [ 1 ] ]

- `TDesignBlockMultiplicityBound(` t`,` v`,` k`,` lambda`)`

  Given a non-negative integer t, and positive integers v, k and lambda, with tleklev, this function returns a non-negative integer which is an upper bound on the multiplicity of any block in any t-(v,k,lambda) design (the multiplicity of a block in a block design is the number of times that block occurs in the block list). In particular, if the value 0 is returned, then this implies that a t-(v,k,lambda) design does not exist.

  Although our bounds are reasonably good, we do not claim that the returned bound m is always achieved; that is, there may not exist a t-(v,k,lambda) design having a block with multiplicity m.

  See also ResolvableTDesignBlockMultiplicityBound.

      gap> TDesignBlockMultiplicityBound(5,16,7,5);
      2
      gap> TDesignBlockMultiplicityBound(2,36,6,1);
      0
      gap> TDesignBlockMultiplicityBound(2,36,6,2);
      2
      gap> TDesignBlockMultiplicityBound(2,15,5,2);
      0
      gap> TDesignBlockMultiplicityBound(2,15,5,4);
      2
      gap> TDesignBlockMultiplicityBound(2,11,4,6);
      3

- `ResolvableTDesignBlockMultiplicityBound(` t`,` v`,` k`,` lambda`)`

  A resolution of a block design is a partition of the blocks into subsets, each of which forms a partition of the point set, and a block design is resolvable if it has a resolution.

  Given a non-negative integer t, and positive integers v, k and lambda, with tleklev, this function returns a non-negative integer which is an upper bound on the multiplicity of any block in any resolvable t-(v,k,lambda) design (the multiplicity of a block in a block design is the number of times that block occurs in the block list). In particular, if the value 0 is returned, then this implies that a resolvable t-(v,k,lambda) design does not exist.

  Although our bounds are reasonably good, we do not claim that the returned bound m is always achieved; that is, there may not exist a resolvable t-(v,k,lambda) design having a block with multiplicity m.

  See also TDesignBlockMultiplicityBound.

      gap> ResolvableTDesignBlockMultiplicityBound(5,12,6,1);
      1
      gap> ResolvableTDesignBlockMultiplicityBound(2,21,7,3);
      0
      gap> TDesignBlockMultiplicityBound(2,21,7,3);
      1
      gap> ResolvableTDesignBlockMultiplicityBound(2,12,4,3);
      1
      gap> TDesignBlockMultiplicityBound(2,12,4,3);
      2

  ## 2.2 Information from (mixed) orthogonal array parameters

  For integers N,k,s,t, with N,k>0, s>1, and 0letlek, an orthogonal array indexorthogonal array OA(N,k,s,t) is an Ntimesk array, in which the entries come from a set of size s of symbols, with the property that in any Ntimest subarray, every possible t-tuple of symbols occurs as a row equally often (which must be N/st times).

  For integers N,k1,...,kw,s1,...,sw,t, with w,N,k1,...,kw>0, s1,...,sw>1, and 0letlek:=k1+cdots+kw, a mixed orthogonal array OA(N,s1k_1,...,swk_w,t) is an Ntimesk array, in which the entries in the first k1 columns come from a set of symbols of size s1, the entries in the next k2 columns come from a set of symbols of size s2, and so on, with the property that in any Ntimest subarray, every possible t-tuple of symbols occurs as a row equally often. Clearly, a mixed orthogonal array OA(N,sk,t) is the same thing as an orthogonal array OA(N,k,s,t).

  The rows of an orthogonal array or mixed orthogonal array are called runs. The multiplicity of a run is the number of times it appears as a row in the array.

- `OARunMultiplicityBound(` N`,` k`,` s`,` t`)`

  Suppose N, k, s, and t are integers, with N, k positive, s > 1, and 0letlek. Then this function returns an upper bound on the multiplicity of any run in an orthogonal array OA(N,k,s,t).

  An upper bound on the multiplicity of a run in a mixed orthogonal array can be obtained by replacing k and s by non-empty lists of the same length, w say, of positive integers, such that 0let le `Sum(`k`)`, and each entry of s is at least 2. Then the function returns an upper bound on the multiplicity of any run in a mixed orthogonal array OA(N,s[1]k[1],...,s[w]k[w],t).

  If the value 0 is returned, then this implies that an orthogonal array or mixed orthogonal array with the given parameters does not exist.

  We do not claim that the returned upper bound m is achieved; that is, there may well be no (mixed) orthogonal array with the given parameters having a run with multiplicity m.

      gap> OARunMultiplicityBound(81,14,3,3);
      1
      gap> OARunMultiplicityBound(81,15,3,3);
      0
      gap> OARunMultiplicityBound(36,[18,1,1],[2,3,6],2);
      1
      gap> OARunMultiplicityBound(72,7,6,2);
      2
      gap> OARunMultiplicityBound(72,8,6,2);
      1

  ## 2.3 Block intersection polynomials

  In CaSo, Cameron and Soicher introduce block intersection polynomials and their applications to the study of block designs. Here we give functions to construct and analyze block intersection polynomials.

- `BlockIntersectionPolynomial(`x`,` m`,` lambdavec`)`

  For k a non-negative integer, define the polynomial P(x,k)=x(x-1)cdots(x-k+1). Let s and t be non-negative integers, with sget, and let m0,...,ms and lambda0,...,lambdat be rational numbers. Then the block intersection polynomial for the sequences [m0,...,ms], [lambda0,...,lambdat] is defined to be

  sumj=0ttchoosejP(-x,t-j)[P(s,j)lambdaj-sumi=js P(i,j)mi],

  and is denoted by B(x,[m0,...,ms],[lambda0,...,lambdat]).

  Now suppose x is an indeterminate over the rationals, and m and lambdavec are non-empty lists of rational numbers, such that the length of lambdavec is not greater than that of m. Then this function returns the block intersection polynomial B(x,m,lambdavec).

  The importance of a block intersection polynomial is as follows. Let D=(V,calB) be a block design, let SsubseteqV, with s=|S|, and for i=0,...,s, suppose that mi is a non-negative integer with mileni, where ni is the number of blocks intersecting S in exactly i points. Let t be a non-negative even integer with tle s, and suppose that, for j=0...,t, we have lambdaj=1/schoose jsumTsubseteqS,|T|=jlambdaT, where lambdaT is the number of blocks of D containing T. Then the block intersection polynomial B(x)=B(x,[m0,...,ms],[lambda0,...,lambdat]) is a polynomial with integer coefficients, and B(n)ge0 for every integer n. (These conditions can be checked using the function BlockIntersectionPolynomialCheck.) In addition, if B(n)=0 for some integer n, then mi=ni for inotin{n,n+1,...,n+t-1}.

  For more information on block intersection polynomials and their applications, see CaSo and Soi10.

      gap> x:=Indeterminate(Rationals,1);
      x_1
      gap> m:=[0,0,0,0,0,0,0,1];;
      gap> lambdavec:=TDesignLambdas(6,14,7,4);
      [ 1716, 858, 396, 165, 60, 18, 4 ]
      gap> B:=BlockIntersectionPolynomial(x,m,lambdavec);
      1715*x_1^6-10269*x_1^5+34685*x_1^4-69615*x_1^3+84560*x_1^2-56196*x_1+15120
      gap> Factors(B);
      [ 1715*x_1-1715,
        x_1^5-1222/245*x_1^4+3733/245*x_1^3-6212/245*x_1^2+5868/245*x_1-432/49 ]
      gap> Value(B,1);
      0

- `BlockIntersectionPolynomialCheck(`m`,` lambdavec`)`

  Suppose m is a list of non-negative integers, and lambdavec is a list of non-negative rational numbers, with the length of lambdavec odd and not greater than the length of m.

  Then, for x an indeterminate over the rationals, this function checks whether `BlockIntersectionPolynomial(`x`,`m`,`lambdavec`)` is a polynomial over the integers and has a non-negative value at each integer. The function returns `true` if this is so; else `false` is returned.

  See also BlockIntersectionPolynomial.

      gap> m:=[0,0,0,0,0,0,0,1];;
      gap> lambdavec:=TDesignLambdas(6,14,7,4);
      [ 1716, 858, 396, 165, 60, 18, 4 ]
      gap> BlockIntersectionPolynomialCheck(m,lambdavec);
      true
      gap> m:=[1,0,0,0,0,0,0,1];;
      gap> BlockIntersectionPolynomialCheck(m,lambdavec);
      false

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