Partially balanced incomplete block design
PBIBD
A (symmetric) association scheme with classes on the
symbols
satisfies:
Two distinct symbols and
are termed
th associates for exactly one
;
each symbol has exactly
th associates; and
when two distinct symbols and
are
th associates, the number of other symbols that are
th associates of
and also
th associates of
is
, independent of the choice of the
th associates
and
. The matrices
of an
-class association scheme are defined as
, and for
,
is a
-matrix whose entry
is
exactly when
and
are
th associates.
Let be a
-set with a symmetric
-class association scheme defined on it. A partially balanced incomplete block design with
associate classes (or
) is a block design based on
with
sets (the blocks), each of size
, and with each symbol appearing in
blocks. Any two symbols that are
th associates appear together in
blocks of
. The numbers
(
) are the parameters of
. The notation
is also used.
is used for the
incidence matrix of
.
Let be the matrices of an association scheme corresponding to a
. Then
and
. Conversely, if
is a
-matrix which satisfies these conditions and the
are the matrices of an association scheme, then
is the incidence matrix of a
.
It is easily verified that , that
, and that
. A
is a balanced incomplete block design (a BIBD; cf. Block design); also, a
in which
is a BIBD.
There are six types of s, [a3], based on the underlying types of association schemes:
1) group divisible;
2) triangular;
3) Latin-square-type;
4) cyclic;
5) partial-geometry-type; and
6) miscellaneous.
Partition the -set
into
groups each of size
. In a group-divisible association scheme the first associates are the symbols in the same group and the second associates are all the other symbols. The eigenvalues of
are
,
and
, with multiplicities
,
, and
, respectively. A group-divisible partially balanced incomplete block design is singular if
; semi-regular if
,
; and regular if
and
.
Let ,
, and arrange the
elements of
in a symmetrical
-array with the diagonal entries blank. In the triangular association scheme, the first associates of a symbol are those in the same row or column of the array; all other symbols are second associates. The duals of triangular
s are the residual designs of symmetric BIBDs with
. Triangular schemes and generalized triangular schemes are also known as Johnson schemes.
Let and arrange the
symbols in an
array. Superimpose on this array a set of
mutually orthogonal Latin squares (see [a1] and also Latin square) of order
. Let the first associates of any symbol be those in the same row or column of the array or be associated with the same symbols in one of the Latin squares. This is an
-type association scheme. If
, then the scheme is group divisible; if
, then all the symbols are first associates of each other.
Let . A non-group divisible association scheme defined on
is cyclic if
and if the set of
differences of distinct elements of
has each element of
times and each element of
times. The first associates of
are
.
In a partial-geometry-type association scheme, two symbols are first associates if they are incident with a line of the geometry and second associates if they are not incident with a line of the geometry.
See [a2], [a4], [a5] for further information.
References
[a1] | R.J.R. Abel, A.E. Brouwer, C.J. Colbourn, J.H. Dinitz, "Mutually orthogonal latin squares" C.J. Colbourn (ed.) J.H. Dinitz (ed.) , CRC Handbook of Combinatorial Designs , CRC (1996) pp. 111–141 |
[a2] | R.A. Bailey, "Partially balanced designs" N.L. Johnson (ed.) S. Kotz (ed.) C. Read (ed.) , Encycl. Stat. Sci. , 6 , Wiley (1985) pp. 593–610 |
[a3] | W.H. Clatworthy, "Tables of two-associate-class partially balanced designs" , Applied Math. Ser. , 63 , Nat. Bureau of Standards (US) (1973) |
[a4] | D. Raghavarao, "Constructions and combinatorial problems in design of experiments" , Wiley (1971) |
[a5] | D.J. Street, A.P. Street, "Partially balanced incomplete block designs" C.J. Colbourn (ed.) J.H. Dinitz (ed.) , CRC Handbook of Combinatorial Designs , CRC (1996) pp. 419–423 |
Partially balanced incomplete block design. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Partially_balanced_incomplete_block_design&oldid=17069