%進退法求函數最小值區間
%宋斌
a0=5;
h0=0.001;
a1=a0;
h=h0;
y1=a1^2-1;
a2=a1+h;
y2=a2^2-1
while(1)
    if y2>y1
        h=-h;
        a3=a1
        y3=y1
    end
    if y3>y1
        a1=a2;
        y1=y2;
        a2=a3;
        y2=y3
        a3=a2+h
        y3=a3^2-1
        if y3>y2
            break
        end
    end
end