function [a,b]=StepOrBack(FUN,x1,h0,flag)
h=h0;
y1=feval(FUN,x1);
x2=x1+h;
y2=feval(FUN,x2);
if y2>=y1
h=-h;
x3=x1;
y3=y1;
x1=x2;
y1=y2;
x2=x3;
y2=y3;
end
h=2*h;
x3=x2+h;
y3=feval(FUN,x3);
while y2>=y3
x1=x2;
y1=y2;
x2=x3;
y2=y3;
h=2*h;
x3=x2+h;
y3=feval(FUN,x3);
end
if h<0
a=x3;
b=x1;
else
a=x1;
b=x3;
end
% all the lines below are designed to show whether the root range result of the specified
% function is right or no
% which means all lines below you can delete them all
% frmluck 2007,10,8
if flag
x0=a:abs(0.1*h):b;
[m,n]=size(x0);
for i=1:n
y0(i)=feval(FUN,x0(i));
end
str=sprintf('range of single root [%.4f,%.4f] and step=%.2f',a,b,h);
plot(x0,y0,'LineWidth',2);
title(str)
grid on
end