function [ na, diag, a ] = r8ss_dif2 ( n )
%*****************************************************************************80
%
%% R8SS_DIF2 sets up an R8SS second difference 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:
%
% 30 June 2016
%
% 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 2*N-1.
%
% Output, integer DIAG(N), the indices in A of the N diagonal
% elements.
%
% Output, real A(2*N-1), the R8SS matrix.
%
diag = zeros ( n, 1 );
a = zeros ( 2 * n - 1 );
na = 0;
for j = 1 : n
if ( 1 < j )
na = na + 1;
a(na) = -1.0;
end
na = na + 1;
a(na) = 2.0;
diag(j) = na;
end
return
end