% A1=mkA1(J,mu,theta)
%
% Make the matrix for the implicit part of the theta method.

function A1=mkA1(J,mu,theta)
    A1=zeros(J-1);
    for j=1:J-1
        A1(j,j)=1+2*mu*theta;
    end
    for j=1:J-2
        A1(j,j+1)=-mu*theta;
        A1(j+1,j)=-mu*theta;
    end
