function [ na, diag, a ] = r8ss_indicator ( n )
%*****************************************************************************80
%
%% R8SS_INDICATOR sets up a R8SS indicator matrix.
%
% Discussion:
%
% The R8SS storage format is used for real symmetric skyline matrices.
% This storage is appropriate when the nonzero entries of the
% matrix are generally close to the diagonal, but the number
% of nonzeroes above each diagonal varies in an irregular fashion.
%
% In this case, the strategy is essentially to assign column J
% its own bandwidth, and store the strips of nonzeros one after
% another. Note that what's important is the location of the
% furthest nonzero from the diagonal. A slot will be set up for
% every entry between that and the diagonal, whether or not
% those entries are zero.
%
% A skyline matrix can be Gauss-eliminated without disrupting
% the storage scheme, as long as no pivoting is required.
%
% The user must set aside ( N * ( N + 1 ) ) / 2 entries for the array,
% although the actual storage needed will generally be about half of
% that.
%
% Licensing:
%
% This code is distributed under the MIT license.
%
% Modified:
%
% 19 March 2004
%
% Author:
%
% John Burkardt
%
% Parameters:
%
% Input, integer N, the order of the matrix.
% N must be positive.
%
% Output, integer NA, the dimension of the array A, which for this
% special case will be the maximum, ( N * ( N + 1 ) ) / 2
%
% Output, integer DIAG(N), the indices in A of the N diagonal elements.
%
% Output, real A((N*(N+1))/2), the R8SS matrix.
%
fac = 10^( i4_log_10 ( n ) + 1 );
na = 0;
for j = 1 : n
for i = 1 : j
na = na + 1;
a(na) = fac * i + j;
end
diag(j) = na;
end
return
end