function Sep18()

% If |f''(x)| <= M2, then the linear interpolant L(x) of (c,f(c)) and (d,f(d))
% satisfies
%                |L(mid) - f(mid)| <= (M2/8)(d-c)^2
% It follows that if
%                 |d-c|^2 M2/2 <= delta
% then we will accept the linear interpolant on [c,d]. Thus,
% sqrt(2*delta/M2)/2 is a lower bound for the subinterval length.