function A = bldA(K,nbc,jmax,bc)
A = eye(K)*-4;
for k = 1:K % for each row
    if k-1 > 0  % node at left arm
        A(k,k-1) = 1;
    end
    if k+1 < K  % node at right arm
        A(k,k+1) = 1;
    end
    if k-jmax > 0   % node at upper arm
        A(k,k-jmax) = 1;
    end
    if k+jmax < K   % node at lower arm
        A(k,k+jmax) = 1;
    end
end
for i = 1:nbc % boundary condition enforcement for A
    m = bc(i,1);
    A(m,:) = 0; A(m,m) = 1; 
end
return;